Nitride semiconductor device with improved lifetime and high output power

ABSTRACT

In the nitride semiconductor device of the present invention, an active layer  12  is sandwiched between a n-type nitride semiconductor layer  11  and an p-type nitride semiconductor layer  13.  The active layer  12  has, at least, a barrier layer  2   a  having an n-type impurity; a well layer  1   a  made of a nitride semiconductor that includes In; and a barrier layer  2   c  that has a p-type impurity, or that has been grown without being doped. An appropriate injection of carriers into the active layer  12  becomes possible by arranging the barrier layer  2   c  nearest to the p-type layer side.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride semiconductor device whichuses a nitride semiconductor (In_(X)Al_(Y)Ga_(1-X-Y)N, 0≦X, 0≦Y, X+Y≦1)used in light emitting devices such as light emitting diode device (LED)and laser diode device (LD), light receiving devices such as solar celland optical sensor or electronic devices such as transistor and powerdevices, and particularly to a nitride semiconductor device comprisingnitride semiconductor layer which includes In.

2. Description of the Prior Art

Recently semiconductor laser devices which use nitride semiconductorhave been receiving increasing demands for the applications in opticaldisk systems such as DVD which are capable of recording and reproducinga large amount of information with a high density. Accordingly, vigorousresearch efforts are being made in the field of semiconductor laserdevice which uses nitride semiconductor. Because of 0the capability tooscillate and emit visible light over a broad spectrum ranging fromultraviolet to red, the nitride semiconductor laser, device is expectedto have wide applications such as light sources for laser printer andoptical network, as well as the light source for optical disk systems.The applicant of the present invention reported a laser whichsuccessfully underwent over ten thousand hours of operation under theconditions of continuous oscillation at a wavelength of 405 nm withoutput power of 5 mW at the room temperature.

Light emitting devices and light receiving devices which use nitridesemiconductor have such a structure as a nitride semiconductor whichincludes In-is used for the active layer and, accordingly, it isimportant to form a better active region in the active layer in order toimprove the device characteristics.

In the prior art, n-type nitride semiconductors doped with n-typeimpurities have beer used for the active layer of the nitridesemiconductor device. Particularly in the case of a device of quantumwell structure, the n-type nitride semiconductors doped with n-typeimpurities have been used in the well layer and, the barrier layer.

In order for light emitting devices which employ nitride semiconductorsto have applications in wide fields, they must be further improved inthe device characteristics, particularly in the device lifetime.

It is essential to have a longer lifetime and a higher output power forthe laser devices which use nitride semiconductors in order to be usedas the light source for reading or writing information in high-densityoptical disk systems described above and have further applications.Other classes of the nitride semiconductor device are also required tohave a longer lifetime and a higher output power, and light emittingdevices are required to have a higher output power of light emission.

Weak reverse withstanding voltage of the devices using nitridesemiconductor, which has been a problem in the part art, has a highprobability of leading to destruction of the device during handling inthe manufacturing process and mounting on an end product, and istherefore one of the most important problems.

The present invention has been made in consideration of the problemsdescribed above, and aims at obtaining a nitride semiconductor devicewhich has excellent device characteristics including the thresholdcurrent density and has longer device lifetime and high output power.

SUMMARY OF THE INVENTION

(1) A light emitting device according to the present invention is a typeof nitride semiconductor device having a structure where an active layerof a quantum well structure, which comprises a well layer made of anitride semiconductor that includes In, and a barrier layer made of anitride semiconductor, is sandwiched by a p-type nitride semiconductorlayer and an n-type nitride semiconductor layer, wherein the lightemitting device according to the present invention is characterized inthat the above active layer has a first barrier layer, that is arrangedin a position nearest to the above p-type nitride semiconductor layer,and a second barrier layer, that is different from the first barrierlayer, as the above barrier layer and is characterized in that the abovefirst barrier layer does not substantially include an n-type impuritywhile the above second barrier layer includes an n-type purity. Here,though, barrier layers, other than the first barrier layer and thesecond barrier layer among the barrier layers in the active layer, arenot particularly limited, in the case of usage as a laser device or as alight emitting device of high power, they are preferably doped with ann-type impurity or are not doped with any impurities.

Though, in a conventional multiple quantum well-type (hereinafterreferred to as MQW-type) nitride semiconductor device, all the barrierlayers are, in general, doped with an n-type impurity, such as Si, inorder to enhance light emission efficiency by increasing the initialelectron concentration in the active layer, a nitride semiconductordevice of the present invention has a barrier layer, that is doped withan n-type impurity in the same manner as in the prior art, while ann-type impurity is not substantially included only in the first barrierlayer that is nearest to the p-type nitride semiconductor layer. In sucha structure, characteristics with respect to the device lifetime and thereverse withstanding voltage of the nitride semiconductor device can beimproved.

Though the mechanism where the lifetime characteristic is improved isnot necessarily evident, it can be inferred that, for one reason, thefact that the lifetime of the carriers has become longer than in theprior art contributes to this mechanism. Conventionally a barrier layer,that is doped with an n-type impurity, is arranged on the side of thep-type layer so that diffusion of the p-type impurity from the p-typelayer occurs to quite a great degree and, thereby, a barrier layer, thatincludes an n-type impurity and a p-type impurity, is provided, whichcauses the lowering of the lifetime of the carriers. According to thepresent invention, since the first barrier layer is not doped with ann-type impurity, n-type and p-type impurities can be prevented fromcoexisting in the same barrier layer.

In addition, among barrier layers in the active layer, the barrier layerarranged on the side of the p-type layer (first barrier layer) does notsubstantially include an n-type impurity so as to have a functiondifferent from that of the barrier layer (second barrier layer), whichhas an n-type impurity, in the active layer. That is to say, by havingthe second barrier layer, the carriers injected from the n-type layerinto the active layer are increased and the carriers that reach deepinto the active layer (to the p-type layer side) are increased so thatthe injection efficiency of the carriers can be increased while, byhaving the first barrier layer, a barrier layer, in which an n-typeimpurity is not included, is arranged as a barrier layer nearest to thep-type layer in the active layer so that it becomes possible to increasethe injection of the carriers from the p-type layer and also to improvethe efficiency.

In the case that an n-type impurity is included in the first barrierlayer, the injection of the carriers from the p-type layer tends to beblocked. In particular, the diffusion distance of the carriers from thep-type layer tends to be short in comparison with the carriers from then-type layer and, therefore, when the first barrier layer, whichcorresponds to the entrance for the injection of the carriers from thep-type layer to the active layer, has an n-type impurity, the injectionof the carriers from the p-type layer is negatively affected to aserious degree. As shown in FIG. 14, it is understood that the devicelifetime is suddenly lowered as the n-type impurity concentration in thefirst barrier layer is increased.

Accordingly, by providing the first barrier layer in the active layer,it is observed that a great number of holes can be provided and thelifetime of the carriers tends to become longer such that they areconsidered to contribute to the increase of the above characteristics.

Though the second barrier layer may adjoin the first barrier layer, itis preferably provided at a distance away from the first barrier layerby making at least one, or more, well layer intervene. Thereby, thefirst barrier layer arranged on the p side and the second barrier layerarranged on the n side are provided with a well layer placed betweenthem within the active layer so that an effective carrier injectionbecomes possible so as to reduce the loss in a laser device as a lightsource for, for example an optical disk system, and the devicecharacteristics, in particular the device lifetime and the power, aresubsequently increased. At this time, the second barrier layer ispreferably a barrier layer nearest to the n-type layer among the barrierlayers in the active layer so as to be the entrance for the injection ofthe carriers from the n-type layer so that a great amount of carrierinjection or an effective injection becomes possible and the devicecharacteristics are improved.

Here, the fact that an n-type impurity is not substantially includedindicates that an n-type impurity is not included due to the exceedingof the concentration resulting from the contamination, or the like,during the process and, for example, in the case that the n-typeimpurity is Si, the fact indicates that the concentration is 5×1016cm−3, or less.

(2) It is preferable for the film thickness of the above first barrierlayer to be greater than the film thickness of the second barrier layer.In this structure increase of the device lifetime can be implemented. Inthe case that the first barrier layer has a film thickness less thanthat of the other barrier layer (second barrier layer), lowering of thedevice lifetime can be observed. In particular, this tendency issignificant in the case that the first barrier layer is arranged in theoutermost position. In addition, in the case that the first barrierlayer is positioned in the outermost position in the active layer, thatis to say, on the top, when a p-type nitride semiconductor layer isprovided on the active layer, the reduction of the above device lifetimeis furthered. For example, as shown in FIG. 8, in the case that thefirst harrier layer 2 c is arranged nearest to the p-type electronconfining layer (first p-type nitride semiconductor layer), the firstbarrier layer becomes an important layer where the film Thicknessthereof determines the characteristics of the active layer and the welllayer since this p-type electron confining layer is a layer thatstrongly affects the active layer, particularly the well layer, asdescribed below.

That is to say, in the nitride semiconductor device according to thepresent invention, carriers can be effectively confined in the activelayer when the barrier layers in the active layer are formed of anitride semiconductor that includes In and the layer, at least,adjoining the active layer among p-type nitride semiconductor layers isformed of a nitride semiconductor (electrons confining layer) thatincludes Al. However, when a nitride semiconductor that includes Al ismade to grow after a nitride semiconductor that includes In is made togrow, the nitride semiconductor that includes In is easily resolvedbecause of the high vapor pressure of InN and because of the differencein the growth conditions of these nitride semiconductors. Therefore, itis preferable for the first barrier layer to be formed thicker than theother barrier layers.

For example, in the case of the growth by means of an MCCVD method, itis general that InGaN is made to grow under the conditions of a slow gasflow rate at a low temperature in a nitrogen gas atmosphere while AlGaNis made to grow under the conditions of a fast gas flow rate at a hightemperature in a hydrogen gas atmosphere. Accordingly, for example,when, after growing InGaN as, the first barrier layer, AlGaN is made togrow as a p-type nitride semiconductor layer, InGaN is resolved througha gas etching at the time when the growth condition within the reactionvessel is switched to another condition. Therefore, by forming the firstbarrier layer thicker than the other barrier layers, an excellentquantum well structure can be maintained even in the case that the firstbarrier layer is slightly resolved. That is to say, the first barrierlayer plays the role of a protective layer that prevents the activelayer, which includes In, from being resolved.

Furthermore, in the case that the first barrier layer arranged nearestto the p-type nitride semiconductor layer has a great film thickness,the distance vis-à-vis the p-type electron confining layer can beincreased so that carriers of a high concentration can be stablyinjected in the continuous drive of the device since a sufficientlybroad space can be secured even for a great amount of p-type carriers.Accordingly, device reliability, such as a long device lifetime, can beimproved.

(3) In addition, when the barrier layer arranged in the position nearestto the n-type nitride semiconductor layer is assumed to be a barrierlayer B1 and the i-th (i=1, 2, 3 . . . L) barrier layer counted from thebarrier layer B1 toward the above p-type nitride semiconductor layer isassumed to be a barrier layer B1, it is preferable for barrier layers Bifrom i=1 to i=n (1<n<L) to have an n-type impurity. Because of thisstructure the injection of the carriers to each well layer in the activelayer becomes more efficient. In addition, the injection of the carriersdeep into the active layer (p-type layer side) is effectively carriedout so that the device can deal with a great amount of carrierinjection. Accordingly, the light emission efficiency is improved, forexample, in an LED or in a LD and it becomes possible to lower theoscillation threshold current density and the forward direction voltagewhile increasing the device lifetime. In addition, the provision of ann-type impurity in the barrier layers bi from the first to the n-thcontributes to the lowering of the threshold current density because thecarriers are immediately injected into the well layers at the initialphase of the drive of the device.

(4) In addition, all of the barrier layers, other than the first barrierlayer, are preferably doped with an n-type impurity. Thereby, thecarrier injection from the n-type layers can further be increased andcan be made more effective.

(5) It is preferable for the above first barrier layer to be arranged inthe outermost position of the above active layer. The first barrierlayer is arranged on the side nearest to the p-type nitridesemiconductor layer within the active layer so that the first barrierlayer becomes the entrance for the injection of the carriers and,thereby, the injection of the carriers from the p-type layer to theactive layer becomes effective and a great amount of carriers can beinjected so as to improve device characteristics, such as the thresholdcurrent density, the device lifetime and power. In addition, a nitridesemiconductor device can be gained which has the device reliability thatcan withstand severe drive conditions, such as a great amount of currentor a high power. At this time, it is preferable for the p-type nitridesemiconductor layer to be formed so as to contact the active layer andthe below described first p-type nitride semiconductor layer can beprovided as a layer that contacts the first barrier layer.

(6) Furthermore, it is preferable for the above second barrier layer tobe arranged in the outermost position close to the above n-type nitridesemiconductor layer within the above active layer. In this structure,the active layer is provided wherein the first p side barrier layer andthe second n side barrier layer are respectively arranged on the p-typenitride semiconductor layer side and on the n-type nitride semiconductorlayer side so that the carriers from the p-type layer and n-type layerare effectively injected toward the center portion of the active layer.

(7) It is preferable in the above structure (6) for the film thicknessof the above first p side barrier layer to be approximately the same asthe film thickness of the above second n side barrier layer. In thisstructure, the active layer becomes more symmetrical and, as a result,the dispersion of the devices can be restrained so as to increase theyield and the threshold current density is reduced.

(8) In addition, it is preferable in the above structure (6) for theabove active layer to have two, or more, well layers so as to have athird barrier layer between these well layers and it is also preferablefor the film thickness of the above third barrier layer to be less thanthe film thicknesses of the above first p side barrier layer and of thesecond n side barrier layer. In this structure, it becomes possible forthe second n side barrier layer and the first p side barrier layer, aswell as the third barrier layer, to have different functions so that itbecomes possible to restrain the dispersion of the devicecharacteristics and to reduce the threshold current density Vf. That isto say, the second n side barrier layer and the first p side barrierlayer are arranged in the outermost position in the active layer so asto be the entrances for the injections of the carriers from the n-typelayer and p-type layer while the film thickness is greater than thethird barrier layer so that a broad space for holding a great amount ofcarriers is secured and, contrarily, the film thickness of the thirdbarrier layer is small so that the film thickness of the entirety of theactive layer can be reduced so as to contribute to the reduction of Vf.

(9) It is preferable for at least one well layer within the above activelayer to have a film thickness of 40 Å, or more. Conventionally, thefilm thickness of the well layer is regarded as optimal in a preferablerange of from approximately 20 Å to 30 Å since the characteristics (forexample, oscillation threshold current) at the initial stage ofoscillation and the light emission are taken into consideration whichresults in the fact that a continuous drive with a great currentaccelerates the device deterioration and prevents the increase of thedevice lifetime. The present invention solves this problem due to theabove structure.

That is to say, the structure of the present invention makes aneffective carrier injection possible and, additionally, by providing awell layer of which the film thickness is suitable for the carrierinjection, it becomes possible to increase the stability in the drive ofa light emitting device and a laser device of high power and loss inoutput, relative to the injected current, can be reduced so that a greatincrease in the device lifetime can be made possible. An effective lightemitting recombination, without loss, of the great amount of carriersinjected in the well layer is required for light emission andoscillation at high power and the above structure is suitable forimplementing such light emission and oscillation.

The upper limit of the film thickness of the well layer depends on thefilm thicknesses of the barrier layers and of the active layer and ispreferably 500 Å, or less, though it is not particularly limited tothis. In particular, it is preferably 300 Å, or less, when it is takeninto consideration that a plurality of layers are layered in the quantumwell structure. Furthermore, in the case that the film thickness of awell layer is in the range of no less than 50 Å and no more than 200 Å,it is possible to form, preferably, an active layer in either a multiplequantum well structure or in a single quantum well structure. In thecase of the multiple quantum well structure in particular, the filmthickness of a well layer is preferably in the range of no less than 50Å and no more than 200 Å, since the number of layers (number of pairs ofa well layer and a barrier layer) is increased. In addition, when thefilm thickness of a well layer is in this preferable range, a highreliability of the device and a long lifetime can be gained for lightemission and oscillation with a large amount of current and with a highpower output while, in a laser device, a continuous oscillation at 80 mWbecomes possible and an excellent device lifetime can be implemented ina broad output range such as from 5 mW to 80 mW. At this time, it isnecessary to adopt the above range of film thickness of a well layer forat least one well layer in the case that the active layer has a multiplequantum well structure and preferably the above film thicknesses areadopted for all of the well layers. By doing so, the above describedeffects are gained in each of the well layers so that a light emittingrecombination and a photoelectric conversion efficiency are furtherimproved. By using a nitride semiconductor that includes In, morepreferably InGaN, for a well layer, an excellent device lifetime can begained. At this time, by making the composition ratio x of In in therange of 0<x≦0.3, a well layer of a thick film with a good crystal canbe formed and preferably by making x≦0.2, a plurality of well layers ofthick films with a good crystal structure can be formed so that anactive layer in a good MQW structure can be gained.

(10) The above described first barrier layer preferably has a p-typeimpurity. In this structure, the above described injection of carriersfrom the p-type layer becomes effective and the lifetime of the carrierstends to increase and, as a result, the structure contributes toincreases in the reverse withstanding voltage, the device lifetime andthe output. This is because, as described above, a carrier injectionfrom the p-type layer becomes excellent due to substantially noinclusion of an n-type impurity and, furthermore, it becomes possible toaccelerate further injection of carriers into the active layer by havinga p-type impurity in the first barrier Layer so that a large amount ofcarriers are effectively injected from the p-type layer into the activelayer or deep inside the active layer (n-type layer side) and, thereby,increases in light emitting recombination, photoelectric conversionefficiency and device lifetime and, in addition, an improvement in thecharacteristic of reverse withstanding can be implemented.

(11) In addition, though the concentration of the p-type impurity in thefirst barrier layer is not in particular limited, it is preferable to beno less than 1×1016 cm−3 and no more than 1×1019 cm−3. In the case thep-type impurity concentration is too low, the hole injection efficiencyinto the well layer is lowered, while if it is too high, the carriermobility in the first barrier layer is lowered so as to increase the Vfvalue of the laser.

(12) The first barrier layer of which the p-type impurity concentrationis in such a range is an i-type or a p-type.

(13) The doping of a p-type impurity into the first barrier layer ispreferably carried out through diffusion from the p-type nitridesemiconductor layer after making the undoped first barrier layer growrather than being carried out at the time of the growth of the firstbarrier layer. This is because, in the case that it is carried out atthe time of the growth of the first barrier layer, a p-type impuritydiffuses into an n-type well layer beneath the first barrier layer atthe time when the first barrier layer grows so that the device lifetimecharacteristics is lowered, on the other hand, in the case that thedoping of a p-type impurity is carried out through diffusion, a p-typeimpurity can be doped into the first barrier layer without affecting thewell layer.

(14) In the case that an n-type nitride semiconductor layer, an activelayer and a p-type nitride semiconductor layer are layered in sequencein the device structure, the barrier layer can have a p-type impuritybecause a p-type impurity diffuses from the p-type nitride semiconductorlayer that is made to grow subsequent to the first barrier layer, whichis made to grow without doping.

(15) A nitride semiconductor device of the present invention preferablyhas a laser device structure wherein said p-type nitride semiconductorlayer has an upper clad layer made of a nitride semiconductor thatincludes Al of which the average mixed crystal ratio of x, wherein0<x≦0.05 and wherein said n type nitride semiconductor layer has a lowerclad layer made of a nitride semiconductor that includes Al of which theaverage mixed crystal ratio of x, wherein 0<x≦0.05. A laser devicegained in such a structure can continuously oscillate with the output of5 mW to 100 mW so as to become an LD having device characteristicssuitable for a reading and writing light source in an optical disksystem and makes it possible to implement a long lifetime. By limitingthe average mixed crystal ratio of Al in the clad layer to 0.05, orless, an optical wave guide which makes it possible to control aself-exciting oscillation at the time of a high power output is providedso that a continuous oscillation with a high power output in a stablemanner becomes possible and it also becomes possible to gain an LD foran optical disk light source. Though, conventionally, a nitridesemiconductor of which the average composition of Al in the clad layeris no less than 0.05 and no more than 0.3 is used, in this structureconfinement of light becomes too strong and, thereby, a self-excitingoscillation is generated in a continuous oscillation with a high outputof 30 mW, or more. This self-exciting oscillation is due to thegeneration of a kink in the current-light output characteristics that isgenerated on the low output side in an LD structure which has a stronglight confinement in the longitudinal direction so as to enhance thelight density and such generation of a kink thus becomes disadvantageousas a light source of an optical disk system so that a self-excitingoscillation due to the kink is unstable and leads to dispersion in thedevices. According to the structure of the present invention, an opticalwave guide, of which the refraction difference in a clad layer isreduced, is gained and by using an active layer that is in the abovedescribed range, a large amount of carriers are continuously injected ina stable manner for light emitting recombination in the structure sothat a continuous oscillation can be gained so as to exceed thecompensation for the loss due to the lowering of the light confinementin the clad layer and, moreover, the light emission efficiency withinthe active layer can be enhanced.

It is preferable that said upper clad layer has a p-type conductivityand said lower clad layer has an n-type conductivity, and that saidactive layer has a first barrier layer that is arranged in a positionnearest to said upper clad layer as said barrier layer and a secondbarrier layer that is different from the first barrier layer and, at thesame time, it is preferable that said first barrier layer has a p-typeimpurity and said second barrier layer has an n-type impurity. In such astructure, as described above, injection of carriers from the p-typelayer is carried out in an excellent condition and, as a result, devicecharacteristics, in particular device lifetime, are improved.

(16) The above p-type nitride semiconductor layer preferably contains afirst p-type nitride semiconductor layer so as to adjoin the activelayer so that the first p-type nitride semiconductor layer is made of anitride semiconductor that includes Al. In this structure, as shown inFIGS. 4 to 7, the first p-type nitride semiconductor layer 28 functionsas an electron confining layer and, in particular, makes it possible toconfine a large amount of carriers within the active layer in an LD andan LE with a large current drive for high power output. In addition, inthe relationships between the above first barrier layer, barrier layerBL and the first p side barrier layer, as shown in FIG. 8, the filmthicknesses of these barrier layers determine the distance dB betweenthe first p-type nitride semiconductor layer and a well layer 1 b so asto greatly affect the device characteristics.

In addition, since the first p-type nitride semiconductor layer may growin the form of a thin film, it can be made to grow at a temperaturelower than that for a p-type clad layer. Accordingly, by forming ap-type electron confining layer, resolution of an active layer thatincludes In can be prevented as opposed to the case where a p-type cladlayer is directly formed on the active layer. That is to say, the p-typeelectron confining layer plays the roles of preventing the resolution ofthe active that includes In in the same manner as does the barrier layerof FIG. 1.

(17) The above described first p-type nitride semiconductor layer isprovided so as to contact the barrier layer nearest to the abovedescribed p-type nitride semiconductor layer, and, preferably, is formedof a semiconductor which has been grown by being doped with a p-typeimpurity of which the concentration is higher than that in a barrierlayer in the above active layer. Because of this structure, theinjection of carriers from the p-type layer to the barrier layer (theabove described first barrier layer) closest to the p-type layer becomeseasy to implement. In addition, by doping a p-type impurity of a highconcentration into the first p-type nitride oxide layer, a p-typeimpurity is diffused into the barrier layer so that an appropriatep-type impurity can be added. Thus, since it becomes unnecessary to addan impurity at the time of barrier layer growth, it becomes possible tomake a barrier layer grow with a good crystal structure. In particular,in the case that this barrier layer is a nitride semiconductor thatincludes In, crystal deterioration is great due to impurity addition andthe effect thereof is significant. In addition, in the case that thefirst p-type nitride semiconductor layer is, as described below, anitride semiconductor that includes Al and the Al mixed crystal ratiothereof is higher than the mixed crystal ratio of Al in the p-type cladlayer, it effectively functions as an electron confining layer whichconfines electrons within the active layer and the effects are gainedwherein an oscillation threshold value and driving current are loweredin a large current drive, a high power LD and an LED.

(18) It is preferable for the number of well layers to be in the rangeof from no less than 1 to no more than 3 in the above described activelayers. In this structure it becomes possible to lower the oscillationthreshold value in comparison with the case where the number ofthreshold layers is 4, or more. In addition, at this time by making thefilm thickness of the well layer 40 Å, or more, as described above, abroad space is secured inside of a small number of well layers so thatan effective light emitting recombination becomes possible even in thecase that a large amount of carriers are injected so that this makes anincrease in the device lifetime and an increase in the light emissionoutput possible. In particular, in the case that the film thickness ofthe well layer is 40 Å, or less, and the number of well layers is 4, ormore, a large amount of carriers are infected into each well layer of athin film, in comparison with the above described case, in order to gaina high power LD or LED by driving the well layers with a large currentso that the well layers are driven under severe conditions and devicedeterioration occurs at an early stage. In addition, when the number ofwell layers increases, the carriers are not distributed uniformly but,rather, tend to be distributed unevenly so that the above describeddevice deterioration becomes a critical problem in the case wherein thedevice is driven with a large amount of current under such a condition.In such a structure as described above, the barrier layer that isnearest to the p-type layer side does not include an n-type impurity orhave a p-type impurity, or other barrier layers each do have an n-typeimpurity and, thereby, a large amount of carriers can be injected intothe well layers in a stable manner and, in addition, the well layer ismaintained at the above described film thickness (40 Å, or more) so thatthese closely relate to each other to appropriately work to implement anexcellent device lifetime and a high light emission output in aconsecutive driving of the device.

(19) It is preferable that the above described second barrier layer isarranged so as to be sandwiched by well layers and, the film thicknessratio Rt of the above described well layer to the second barrier layeris in the range of from 0.5≦Rt≦3 Because of this structure a lightemitting device and a laser device can be gained wherein the responsecharacteristics are excellent and RIN is low in order to be usedspecifically for an optical disk system, an optical communicationsystem, and the like. That is to say, though the film thicknesses of thewell layers, the barrier layers and the active layers become factorsthat greatly affect the RIN and the response characteristics in theactive layer of a quantum well structure, a light emitting device and alaser device that are excellent in these characteristics can be gainedby limiting the film thickness ratio of the well layer to the barrierlayer to the above described range in this structure.

(20) It is preferable for the film thickness dw of the above describedwell layer to be in the range of 40 Å≦dw≦100 Å and for the filmthickness db of the above described second barrier layer to be in therange of db≧40 Å. In this structure, by adjusting the film thickness ofthe well layer so that the above described film thickness ratio Rt is inthe above described range, a laser device having a long lifetime, a highpower output, as shown in FIG. 12, and having RIN characteristics aswell as response characteristics that are suitable for the light sourceof an optical disk system. That is to say, in the light emitting deviceof the present invention the lifetime can be made longer by increasingthe film thickness of the well layer while, on the other hand, theresponse characteristics and the RIN characteristics tend to be loweredwhen the film thickness of the well layer is increased. In thisstructure, this is appropriately improved. In addition, in the case thatthe film thickness of the barrier layer is 40 Å, or more, an excellentdevice lifetime can be gained so that a laser device that becomesan-excellent light source in an optical disk system can be gained asshown in FIG. 13.

(21) It is preferable for the above described p-type nitridesemiconductor layer and the above described n-type nitride semiconductorlayer to have, respectively, an upper clad layer and a lower clad layerso that the average mixed crystal ratio of Al in the nitridesemiconductor of the upper clad layers become greater than that in thelower clad layers. This is because, in an effective refraction typelaser device, confinement in the lateral direction can be reduced byincreasing the mixed crystal ratio of Al in the upper clad layer,wherein an effective refraction difference is created, so as to have anupper clad layer of which the refraction is small. That is to say, byreducing the refraction difference between a buried layer, which createsan effective refraction difference on both sides of the wave guide, andthe upper clad layer, the structure can be gained wherein theconfinement in the lateral direction is made smaller. Thus, theconfinement in the lateral direction is reduced and light density isreduced and, thereby, a laser device can be gained wherein, up to a highoutput range, no kink is generated.

(22) Furthermore, the average mixed crystal ratio x of Al in the abovedescribed upper clad layer is in the range of 0<x≦0.1 and, thereby, alaser device can be gained which has the laser characteristics, inparticular, characteristics such as current-light outputcharacteristics, that can be used appropriately for an optical disksystem. At this time the oscillation wavelength of the laser device canbe adjusted in the range of from no less than 380 nm to no more than 420nm so that an appropriate laser device can be gained by using the abovedescribed clad layer.

(23) The above described p-type nitride semiconductor layer has a firstp-type nitride semiconductor layer that contacts the above describedactive layer and becomes an electron confining layer and the activelayer has a well layer of which the distance dB from the first p-typenitride semiconductor layer is in the range of from no less than 100 Åto no more than 400 Å and has a first barrier layer within the distancedB and, thereby, a nitride semiconductor device of which the devicelifetime is excellent can be gained. This can be made to be a devicestructure wherein, as shown in FIG. 8, the distance dB from the firstp-type nitride semiconductor layer 28 has a first barrier layer, that isto say, a barrier layer that does not substantially have an n-typeimpurity or that is adjusted to have a p-type impurity and, thereby,device deterioration due to the first p-type nitride semiconductorlayer, which is a p-type carrier confining layer, is prevented so as toimprove the device lifetime and it becomes possible to accelerate alight emitting recombination in a well layer arranged outside of thedistance dB. Here, the device has, at least, the above described firstbarrier layer in the region of the distance dB, that is to say, thedevice has an impurity adjusted region, wherein, as described above, animpurity, or the amount of the impurity, is adjusted in at least aportion thereof in the region of the distance dB. At this time, thedistance dB is preferably the first harrier layer, that is to say, afirst barrier layer of which the film thickness is dB is preferablyformed so as to contact the first p-type nitride semiconductor layer sothat the above described effects can be maximally gained. In thismanner, by using the region of the distance dB as an impurity adjustedregion, as shown in FIG. 8B, a structure can be gained wherein aplurality of layers of different band gap energies are provided. Forexample, in FIG. 8B a region 4, of which the band gap energy is smallerthan that of the barrier layer 2 c, is formed wherein the abovedescribed effects can be gained by making the region dB an impurityadjusted region. Contrarily, in a similar manner, a layer 4, which hasband gap energy larger than that of the barrier layer 2 b, may beprovided. That is to say, in the case that a plurality of layers ofwhich the band gap energies are different are provided in the region dB,a device of which the characteristics are excellent can be gained byadjusting the impurity, or the amount of the impurity, in the region dBwhich is used as the first barrier layer. Furthermore, the distance dBis preferably in the range of from no less than 120 Å to no more than200 Å so that a nitride semiconductor device with an appropriate activelayer in the device structure can be gained.

As the n-type impurity used in the nitride semiconductor device of thepresent invention, group IV or VI elements such as Si, Ge, Sn, S, O, Tiand Zr may be used, while Si, Ge or Sn is preferable and most preferablySi is used. As the p-type impurity, Be, Zn, Mn, Cr, Mg, Ca or the likemay be used, and Mg is preferably used.

For the purpose of the present invention, the term undoped means anitride semiconductor grown without adding p-type impurity or n-typeimpurity as a dopant, for example organometallic vapor phase growingprocess in a reaction vessel without supplying any impurity which wouldserve as dopant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing one embodiment of thepresent invention.

FIG. 2 is a schematic sectional view showing one embodiment of thepresent invention.

FIG. 3 is a schematic sectional view showing one embodiment of thepresent invention.

FIG. 4 is a schematic sectional view of stacked structure and schematicdiagram showing band structure according to one embodiment of thepresent invention.

FIG. 5 is a schematic sectional view of stacked structure and schematicdiagram showing band structure according to one embodiment of thepresent invention.

FIG. 6 is a schematic sectional view of stacked structure and aschematic diagram showing band structure according to one embodiment ofthe present invention.

FIG. 7 is a schematic sectional view of stacked structure and aschematic diagram showing band structure according to one embodiment ofthe present invention.

FIGS. 8A and 8B is a schematic sectional view of stacked structure andschematic diagram showing band structure according to one embodiment ofthe present invention.

FIGS. 9A and 9B is a schematic sectional view of a device according toone embodiment of the present invention.

FIG. 10 is a schematic sectional view of stacked structure and schematicdiagram showing band structure according to one embodiment of thepresent invention.

FIG. 11 is a schematic sectional view showing one embodiment of thepresent invention.

FIG. 12 is a diagram showing the relationship between the devicelifetime and well layer thickness in one embodiment of the presentinvention.

FIG. 13 is a diagram showing the relationship between the devicelifetime and barrier layer thickness in one embodiment of the presentinvention.

FIG. 14 is a diagram showing the relationship between the devicelifetime and doping concentration in one embodiment of the presentinvention.

FIG. 15 is a diagram showing the relationship between the reversewithstanding voltage and doping concentration in one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The nitride semiconductor used in the nitride semiconductor device ofthe present invention may be GaN, AlN or InN, or a mixed crystalthereof, namely gallium nitride compound semiconductor(In_(x)Al_(y)Ga_(1-x-y)N, 0≦x, 0≦y, x+y≦1). Mixed crystals made by usingB as the group III element or by partially replacing N of the group Velement with P or As may also be used.

(Active Layer)

The active layer of the present invention has quantum well structurewhich may be either multiple quantum well structure or single quantumwell structure, but Preferably multiple quantum well structure whichmakes it possible to increase the output power and decrease thethreshold of oscillation. The quantum well structure of the active layermay be constituted from well layers and barrier layers to be describedlater being stacked one on another. This structure of stacked layers issuch that the well layer is sandwiched by the barrier layers. In thecase of single quantum well structure, at least one barrier layer isprovided each on the p-type nitride semiconductor layer side and on then-type nitride semiconductor layer side so as to sandwich the welllayer. In the case of multiple quantum well structure, the active layercomprising a plurality of well layers and barrier layers stacked one onanother is constituted as described later in preferred embodiments.

The active layer has preferably such a structure as barrier layers areprovided as layers located at positions nearest to the n-type nitridesemiconductor layer and the p-type nitride semiconductor layer(hereinafter referred to as outermost layer). The outermost layerslocated on both sides are preferably barrier layers.

In the multiple quantum well structure, the barrier layer sandwichedbetween the well layers is not limited to a single layer (welllayer/barrier layer/well layer), and two or more barrier layers ofdifferent compositions and/or impurity concentrations may be stackedsuch as well layer/barrier layer (1)/barrier layer (2) . . . /welllayer. For example, such a structure may also be employed as an upperbarrier layer 403 made of a nitride semiconductor including Al and alower barrier layer 402 having an energy band gap smaller than that ofthe upper barrier layer are provided between the well layers 401, asshown in FIG. 10.

(Well Layer)

The well layer of the present invention is preferably made of a nitridesemiconductor including In, having a composition of (In_(α)Ga_(1-α)N(0<α≦1). This constitution makes the well layer capable of oscillatingand emitting light satisfactorily. Wavelength of the emitted light canbe determined by controlling the proportion of In. In addition to InGaN,the nitride semiconductors described above such as InAlGaN and InN maybe used and nitride semiconductors which do not include In may also beused in the present invention, but a nitride semiconductors whichincludes In as higher efficiency of light emission and is morepreferable.

Thickness and number of the well layers may be determined as requiredexcept for the case to be described later in the fifth embodiment.Specifically, thickness of the well layer is in a range from 10 Å to 300Å, and preferably in a range from 20 Å to 200 Å, which allows it todecrease Vf and the threshold current density. When the crystal growthis taken into consideration a layer of relatively homogeneous qualitywithout significant variations in the thickness can be obtained when thethickness is 20 Å or greater, and the crystal can be grown whileminimizing the generation of crystal defects by limiting the thicknesswithin 200 Å. There is no limitation on the number of the well layersprovided in the active layer, which may be 1 or more. When four or morewell layers with larger thickness of layers constituting the activelayer, total thickness of the active layers becomes too large and thevalue of Vf increases. Therefore, it is desirable to restrict thethickness of the well layer within 100 Å thereby to restrain thethickness of the active layer.

The well layer may bc doped or undoped with n-type impurity. When anitride semiconductor which includes In is used as the well layer,however, an increase in the concentration of r-type impurity leads tolower crystallinity and therefore it is preferable to restrict theconcentration of the n-type impurity thereby to make the well layer ofgood crystallinity. Specifically, in order to achieve bestcrystallinity, the well layer is preferably grown without doping, withthe n-type impurity concentration being kept within 5×10¹⁶/cm³. When thewell layer is doped with n-type impurity, controlling the n-typeimpurity concentration within a range from 1×10¹⁸/cm³ to 5×10¹⁶/cm³makes it possible to suppress the degradation of crystallinity andincreases the carrier concentration, thereby decreasing the thresholdcurrent density and the value of Vf. At this time, it is preferable tokeep the n-type impurity concentration in the well layers nearly equalto or less than the n-type impurity concentration in the barrier layer,which will accelerate the light emission recombination in the well layerand increase the light emission output power. Thus the well layers dopedwith the n-type impurity are preferably used in a low output device suchas LD or LED having output power of 5 mW, having effects of decreasingthe value of Vf and the threshold current density. In order to keep then-type impurity concentration in the well layers nearly equal to or lessthan that of the barrier layer, the well layer may be doped with largeramount of n-type impurity than that for the barrier layer when growing,or modified doping may be employed wherein the barrier layer is grownwhile being doped and the well layers are grown without doping. At thistime, the well layers and the barrier layers may be grown without dopingthereby to constitute a part of the active layer.

In a device driven with a large current (such as LD or LED of highoutput power), recombination of the carriers in the well layers isaccelerated and light emission recombination occurs at a higherprobability when the well layers are undoped and do not substantiallyinclude n-type impurity. When the well layers are doped with the n-typeimpurity, in contrast, carrier concentration in the well layersincreases resulting in lower probability of the light emissionrecombination which leads to a vicious cycle of the drive currentincreasing with a constant output power, and consequently causing thedevice reliability (lifetime of the device) to decrease significantly.For this reason, in a high power device (such as LD or LED of outputpower in a range from 5 to 100 mW), the n-type impurity concentration inthe well layers is kept not higher than 1×10¹⁸/cm³, and preferably thewell layers are grown without doping or with such a concentration thatcan be regarded as including substantially no n-type impurity, whichmakes it possible to achieve a nitride semiconductor device capable ofstable operation with high output power. In a laser device with the welllayer being doped with n-type impurity, spectrum width of the peak ofthe laser beam tends to spread and therefore the n-type impurityconcentration is kept within 1×10¹⁸/cm³, and preferably 1×10¹⁷/cm³.Doping n-type impurity in the well layer also degrades the crystallinityof the well layer. If a device with the low-crystallinity well layer isoperated with so high current density as likely in a laser device, thewell and device is degraded and the life of the device is shorten.

(Barrier Layer)

According to the present invention, there is no limitation to thecomposition of the barrier layer, and a nitride semiconductor includingIn with the proportion of In being lower than that of the well layer, ora nitride semiconductor including GaN or Al may be used. Specificcomposition may be In_(β)Al_(γ)Ga_(1-β-γ)N (0≦β≦1, 0≦γ<1),In_(β)Ga_(1-β)N (0≦β<1, α>β, GaN or Al_(γ)Ga_(1-γ)N (0<γ<1) In the caseof a barrier layer (lower barrier layer) which serves as the base layerin contact with the well layer, a nitride semiconductor which does notinclude Al is preferably used. Specifically, In_(β)Ga_(1-β)N (0≦β<1,α>β) or GaN is preferably used. This is because growing a well layermade of a nitride semiconductor which includes In directly on a nitridesemiconductor which includes Al such as AlGaN leads to lowercrystallinity, eventually resulting in impeded function of the welllayer Also it is intended to make the band gap energy of the barrierlayer higher than that of the well layer. Best combination of thecompositions of the well layers and the barrier layers may be selectedfrom the compositions described above, by giving consideration to thefact that the barrier layer serves also as the base layer whichdetermines the crystallinity of the well layer.

The barrier layer may be doped or undoped with the n-type impurity,except for the barrier layer located nearest to the p-type layer to bedescribed later, but preferably doped with the n-type impurity. Whendoped, the n-type impurity concentration in the barrier layer ispreferably 5×10¹⁶/cm³ or higher and lower than 1×10²⁰/cm³. In the caseof LED which is not required to have a high output power, for example,the n-type impurity concentration is preferably in a range from5×10¹⁶/cm³ to 2×10¹⁸/cm³. For LED of higher output power and LD, it ispreferable to dope in a range from 5×10¹⁷/cm³ to 1×10²⁰/cm³ and morepreferably in a range from 1×10¹⁸/cm³ to 5×10¹⁹/cm³. When doping to sucha high concentration, it is preferable to grow the well layer withoutdoping or with substantially no n-type impurity included. The reason forthe n-type impurity concentration being different among the regular LED,the high-power LED and the high-power LD (output power in a range from 5to 100 mW) is that a device of high output power requires higher carrierconcentration in order to drive with larger current for higher outputpower. Doping in the range described above, as described above, it ismade possible to inject the carrier to a high concentration with goodcrystallinity. In the case of a nitride semiconductor device such aslower-power LD, LED or the like, in contrast, a part of the barrierlayer of the active layer may be doped with the n-type impurity or theentire barrier layers maybe formed with substantially no n-type impurityincluded. When doping with the n-type impurity, all the barrier layersof the active layer may be doped or a part of the barrier layers may bedoped.

While there is no limitation to the thickness of the barrier layer, thethickness is preferably not larger than 500 Å, and more specificallyfrom 10 to 300 Å similarly to the well layer.

In the embodiments to be described later, barrier layers doped withp-type impurity are used. The p-type impurity concentration is in arange from 5×10¹⁶/cm³ to 1×10²⁰/cm³, and preferably in a range from5×10¹⁶/cm³ to 1×10¹⁸/cm³. This is because p-type impurity concentrationhigher than 1×10²⁰/cm³ does not cause substantial difference in thecarrier concentration while causing deterioration of the crystallinitydue to inclusion of the impurity and an increase in the loss due tolight scattered by the impurity, thus resulting in lower efficiency oflight emission in the active layer. When impurity concentration iswithin ×10¹⁸/cm³, the lowering of efficiency of light emission due toincreasing impurity can be suppressed and high carrier concentrationfrom the p-type layer into the active layer can be kept stable. Inaddition, a trace of p-type impurity is preferably included as the lowerlimit of the p-type impurity concentration. This is because a low p-typeimpurity concentration causes the p-type impurity to function as acarrier with a higher probability than in the case of highconcentration. At this time, the barrier layer which includes the p-typeimpurity preferably includes substantially no n-type impurity, in theembodiments described later. This is because the barrier layer whichincludes the p-type impurity having substantially no n-type impurityincluded therein functions as a barrier layer that accelerates theinjection of the carrier from the p-type layer, while the presence ofthe p-type impurity enhances the action. FIG. 14 and FIG. 15 show thelifetime of the device and the reverse withstanding voltage,respectively, as the functions of the n-type impurity concentration inthe barrier layer located nearest to the p-type layer. As will beapparent from these graphs, lifetime of the device and the reversewithstanding voltage decrease sharply as the n-type impurityconcentration increases, resulting in deterioration in the devicecharacteristics. Thus in the nitride semiconductor device of the presentinvention, the barrier layers (the first barrier layer, the barrierlayer B₁ and the first p-type barrier layer) which are nearest to thep-type layer are preferably grown without doping with the n-typeimpurity or not doping substantially with the n-type impurity, morepreferably include p-type impurity, and most preferably include p-typeimpurity without including n-type impurity. This is because the absenceof n-type impurity makes the injection of the carrier from the p-typelayer efficient, while the presence of the p-type impurity acceleratesthe injection of the carrier, and the combination of these conditions,namely including p-type impurity without including n-type impurity,makes it possible to inject a large amount of carrier efficiently fromthe p-type layer.

(Doping With n-type Impurity)

According to the present invention, the active layer comprises welllayers which include n-type impurity 5×10¹⁶/cm³ in concentration orhigher and barrier layers, and preferably one or more of the well layersand/or barrier layers in the active layer is undoped or does notsubstantially include n-type impurity. This results in the active layerwhich as whole contains n-type impurity while the well layers and/orbarrier layers which constitute a part of the active layer are dopedwith the r-type impurity, thus achieving an efficient distribution ofcarrier concentration for the active layer.

For the purpose of the present invention, the term undoped means anitride semiconductor grown without intentionally doping with n-type orp-type impurity. At this time, impurity concentration becomes less than5×10¹⁶/cm³. In the present invention, not substantially including n-typeimpurity or p-type impurity means that the impurity concentration isless than 5×10¹⁶/cm³.

Described above are the explanations of the active layer, the barrierlayers and the well layers not detailed in the preferred embodimentsthat follow, and will he complemented by the description of theembodiments.

Embodiment 1

The first embodiment of the nitride semiconductor device according tothe present invention comprises, as shown in FIG. 2 and FIG. 3, anactive layer 12 sandwiched by a p-type nitride semiconductor layer 13and an n-type nitride semiconductor layer 11, with the active layerincluding therein a first barrier layer located at a position nearest tothe p-type nitride semiconductor layer and a second barrier layer whichincludes n-type impurity. The first barrier layer is undoped with then-type impurity, or has been grown without doping so that substantiallyno n-type impurity is included therein. For the first barrier layer,either the layer nearest to the p-type nitride semiconductor layer inthe active layer (hereinafter referred to as the layer nearest to the pside) may be a well layer 1 b as shown in FIG. 2, or this layer may befirst barrier layer as shown in FIG. 3. Preferably, as shown in FIG. 3,when the layer nearest to the p side in the active layer is used thefirst barrier layer, the first barrier layer can be provided in theactive layer in contact with the p-type nitride semiconductor layer, sothat the p-type nitride semiconductor layer 13, the first barrier layer2 d in the active layer 12 and the continuous p-type layer can be formedinto the active layer as shown in FIG. 3. This makes it possible toefficiently inject the carrier from the p-type layer into the activelayer, thereby decreasing the loss in driving the device and improvingthe device characteristics, particularly the reverse withstandingvoltage and the device lifetime. In the case shown in FIG. 3, incontrast, since the well layer 1 b is interposed between the p-typenitride semiconductor layer 13 and the first barrier layer 2 c,continuous p-type layer may not be formed. However, since the firstbarrier layer 2 c is the layer nearest to the p side in the activelayer, an effect not conspicuous as but similar to that of the formercase (the case of FIG. 3) is obtained thus making efficient injection ofthe carrier possible, although the effect is somewhat lower than that ofthe former case (the case of FIG. 3). At this time, the well layerpreferably undoped as described above and, when the n-type impurity isincluded, the concentration thereof is preferably lower than that in thebarrier layer.

Or the other hand, in case the barrier layer which contains the p-typeimpurity (hereinafter referred to as p-type barrier layer) is thebarrier layer not located at the position nearest to the p side in theactive layer, making the barrier layer 2 c a p-type barrier layer inFIG. 3, for example, results in deteriorated device characteristics.This is because the diffusion path length of the p-type carrier (hole)is significantly shorter than that of the n-type so that no substantialcontribution is achieved to the improvement in the efficiency of carrierinjection into the active layer, while injection of the n-type carrieris impeded leading to greater loss. This problem is most conspicuouswhen the barrier layer not located at the position nearest to the p sideincludes the p-type impurity.

The second barrier layer may be provided on the n-type nitridesemiconductor layer side (hereinafter referred to as n side) adjacent tothe first barrier layer, but is preferably provided via at least onewell layer as shown in FIG. 2, FIG. 3. In this constitution, since theactive layer has such a structure that comprises the first barrier layerwhich includes the p-type impurity is located nearest to the p side viaone or more well layer, and the second barrier layer which includes then-type impurity, injection of carrier into one or more well layer beingsandwiched can be made more efficiently than in the case of adjacentarrangement without interposing the well layer. Therefore, it ispreferable that the barrier layer nearest to the n side, namely thebarrier layer 2 a in FIGS. 2, 3, is at least the second barrier layer,that is, the first barrier layer and the second barrier layer are theoutermost barrier layers of the active layer and located on the p sideand on the n side, respectively. Moreover, the second barrier layer maybe provided by single, or may be all the barrier layers except for thefirst barrier layer. Accordingly, in the first embodiment, the barrierlayer nearest to the p side is the first barrier layer and the barrierlayer nearest to the n side is the second barrier layer and morepreferably, in addition to the constitution described above, all thebarrier layers except for the barrier layer nearest to the p side arethe second barrier layer. This constitution makes it possible to injecta large amount of carriers efficiently under high output operation ofthe device, resulting in improved reliability of the device under highoutput power operation. At this time, in addition to the constitution ofthe barrier layer nearest to the p side being the first barrier layerand the barrier layer nearest to the n side being the second barrierlayer, such a constitution may also be possible as the barrier layersecond nearest to the p side or the second nearest to the p side and thesubsequent barrier layers are used as the p-type barrier layers. In theconstitution of the first barrier layer 2 d, the p-type barrier layer 2c and the second barrier layer 2 a in FIG. 3, difference in thediffusion path length of the carrier impedes the improvement in theefficiency of carrier injection and recombination, leading to greaterloss.

The first barrier layer of the first embodiment of the present inventionwill be described in more detail below. It is an important factor inachieving the effect described above that the first barrier layer doesnot substantially include the n-type impurity as well as to include thep-type impurity. That is, similar effect can be achieved by notincluding the n-type impurity as that achieved by including the p-typeimpurity. This is because, since the first barrier layer does notinclude the n-type impurity, it is made possible to inject a largeamount of the carrier efficiently from the p-type layer into the activelayer or into the well layer nearest to the p side in the vicinity ofthe interface between the p-type layer or in the vicinity of the firstbarrier layer in the active layer, thereby improving the devicecharacteristics similarly to the case described above. Conversely,making the first barrier layer virtually void of the n-type impurityleads to the improvement in the device characteristics. Preferably,having the p-type impurity included without substantially including then-type impurity makes the effect described above remarkable. The secondbarrier layer is preferably grown without doping with the p-typeimpurity, or grown so as not to substantially include p-type impurity.

In the first embodiment of the present invention, lifetime of the devicecan be increased by making the thickness of the first barrier layerlarger than the thickness of the second barrier layer. This is becauseas a sufficient space is secured as the first barrier layer where muchp-type impurities exist during high output operation in addition to therelation with the first p-type nitride semiconductor layer to bedescribed later, stable injection of carriers and recombination areensured even with high output power. Conversely, the fact that thesecond barrier layer is thinner than the first barrier layer means thatthe distance of the well layer in the active layer from the n-type laserside is smaller, thereby accelerating the carrier injection from then-type layer side to each well layer. At this time, by providing thesecond barrier layer by single, or setting all the barrier layers exceptfor the first barrier layer as the second barrier layer, the distancesof all the well layer from the n-type layer side can be made smaller,thereby making the carrier injection from the n-type layer sideefficient.

Embodiment 2

In the second embodiment of the present invention, the active layer hasL (L≧2) barrier layers and, with a barrier layer located at a positionnearest to the n-type nitride semiconductor layer denoted as B₁ and abarrier layer which is the ith (i=1, 2, 3, . . . , L) layer from saidbarrier layer B₁ to said p-type nitride semiconductor layer side denotedas barrier layer B_(i), the barrier layers B_(i) of i=1 to i=n (1<n<L)have n-type impurity, while barrier layer B_(L) of i=L does notsubstantially include n-type impurity. The barrier layer B correspondsto the first barrier layer of the first embodiment and is the barrierlayer located nearest to the p side, while the action of the barrierlayer B_(L) is similar to that of the first embodiment. Therefore, thebarrier layer B_(L) in the second embodiment contains at least thep-type impurity and preferably does not substantially include the n-typeimpurity, so that the p-type carrier is injected selectively into thebarrier layer B_(L), thereby making the efficient injection of thecarrier possible. Also because the harrier layers B_(i) of i=1 to i=ninclude the n-type impurity so that n barrier layers are doped with then-type impurity starting with the barrier layer nearest to the n-typelayer thereby increasing the carrier concentration, the carrier isinjected smoothly from the n-type layer into the active layer, resultingin accelerated injection and recombination of the carrier and improveddevice characteristics. At this time, the well layer may be eitherundoped or doped with the n-type impurity. Particularly when the deviceis driven with a large current (high output LD or LED), making the welllayer undoped or not substantially including the n-type impurityaccelerates the recombination of the carrier in the well layer so thatthe nitride semiconductor device of high device characteristics and highreliability can be obtained.

The number n in the second embodiment is required to satisfy at leastthe condition of 0<n<L, and preferably satisfy the condition ofn_(m)<n<L and n_(m)=L/2 (n_(m) is an integer with the fraction roundedoff). This is because it is made possible to inject the carrier from then-type layer deep into the active layer (to the p-type layer side) byhaving the n-type impurity included in half or more of the barrierlayers in the active layer. This acts advantageously particularly in thecase of a multi-quantum well structure wherein the number of the welllayers in the active layer is 3 or more, or the number of layers stackedin the active layer is 7 or more. Specifically, in FIGS. 2, 3, thebarrier layers 2 a, 3 a are doped with the n-type impurity as thebarrier layers B_(i), the barrier layers 2 c (FIG. 2) or the barrierlayers 2 d (FIG. 3) is doped with the p-type impurity as the barrierlayers B_(L), and other barrier layers sandwiched by the barrier layersB₁ and the barrier layers B_(L) are undoped, thereby to constitute theactive layer.

In the second embodiment, by making the barrier layer B_(L) thicker thanthe barrier layer B_(i) (i≠L) as described above, in a high outputdevice which requires stable injection of carriers in a large amountinto the well layer, since the barrier layer B_(L) located at a positionnearest to the p-type layer (near the entrance of injecting the carrierfrom the p-type layer) has a large space where the p-type carriersexist, the carriers of high concentration can be injected in a stablemanner, so that reliability of the device such as lifetime is improved.

Embodiment 3

The active layer 107 has MQW structure wherein In_(x1)Ga_(1-x1)N welllayer (0<x₁<1) and In_(x2)Ga_(1-x2)N barrier layer (0<x₂<1, x₁>x₂) arestacked alternately a proper number of times in the order of barrierlayer, well layer, barrier layer, with both ends of the active layerbeing the barrier layers. The well layers are grown undoped. On theother hand, all barrier layers except for the last barrier layer whichadjoins the p-type electron confinement layer 108 are doped with ann-type impurity such as Si or Sn, while the first barrier layer is grownundoped. The last barrier layer includes a p-type impurity such as Mgwhich has diffused therein from the adjacent p-type nitridesemiconductor layer.

As the barrier layers except for the first barrier layer are doped withthe n-type impurity, the initial electron density in the active layerbecomes higher and the efficiency of injecting electrons into the welllayer is increased, thus resulting in improved efficiency of laseremission. The last barrier layer, on the other hand, is located nearestto the p-type layer and therefore does not contribute to the injectionof electrons into the well layer. Therefore, the efficiency of injectingholes into the well layer can be improved by virtually doping throughthe diffusion of the p-type impurity from the p-type layer, withoutdoping the first barrier layer with the n-type impurity. Also becausethe first barrier layer is not doped with the n-type impurity, such aproblem can be eliminated that the carrier mobility decreases due to thecoexistence of impurities of different types in the barrier layer.

An example of the active layer is described below. The active layer 107is formed on the n-type nitride semiconductor layers 103 to 106. Asdescribed previously, the active layer 107 has the MQW structure whereinIn_(x1)Ga_(1-x2)N well layer (0<x₁<1) and In_(x2)Ga_(1-x2)N barrierlayer (0≦x₂<1, x₁>x₂) are stacked alternately a proper number of times,with both ends of the active layer being the barrier layers. The welllayer is formed undoped. All barrier layers except for the first barrierlayer are doped with an n-type impurity such as Si or Sn preferably in aconcentration from 1×10¹⁷/cm³ to 1×10¹⁹/cm³.

The last barrier layer is grown undoped, and includes a p-type impuritysuch as Mg in a concentration from 1×10¹⁶/cm³ to 1×10¹⁹/cm³ through thediffusion from the p-type electron confinement layer 10B to be grownnext. When growing the first barrier layer, it may also be grown whiledoping with p-type impurity such as Mg in a concentration not higherthan 1×10¹⁹/cm³. The east barrier layer is formed to be thicker than theother barrier layers in order to suppress the effect of decomposition bygas etching when growing the p-type electron confinement layer 108.Thickness of the first barrier layer, is preferably 1.2 to 10 times thethickness of the other barrier layers, more preferably 1.1 to 5 times,although it depends on the conditions of growing, the p-type electronconfinement layer 108. With this constitution, the first barrier layerserves as the protective film which prevents the active layer thatincludes In from decomposing.

Embodiment 4

The fourth embodiment of the present invention has such a constitutionas a first p side barrier layer disposed at a position nearest to thep-type nitride semiconductor layer and a second n side barrier layerwhich is disposed at a position near to the n-type nitride semiconductorlayer are provided as the outermost layers in the active layer, and thefirst p side barrier layer includes p-type impurity while the second nside barrier layer includes n-type impurity. In this constitution, asshown in FIG. 3, the active layer is sandwiched by the first p sidebarrier layer 2 a and the second n side barrier layer 2 d and has thewell layer 1 and the barrier layers 2 b, 2 c. Since the first p sidebarrier layer is provided as the layer disposed at a position nearest tothe p side in the active layer, the carriers can be injected efficientlyfrom the p-type layer, while injection of the carriers from the n-typelayer can be made satisfactorily by providing the second n side barrierlayer as the layer disposed at a position near to the n-type layer inthe active layer. As a result, efficient injection of the carriers fromthe n side layer and the p side layer into the active layer andrecombination thereof are made possible, thus achieving improvements inthe reliability and lifetime of the device of high output power. At thistime, the p-type layer and the n-type layer are preferably provided toadjoin the first p-type barrier layer 2 d and the second n-type barrierlayer 2 a as shown in FIG. 3, which causes the p-type layer and then-type layer to be connected directly to the active layer, therebyachieving better injection of the carriers. At this time, while thebarrier layers sandwiched by the first p side barrier layer 2 d and thesecond n side barrier layer, the barrier layers 2 b, 2 c in the case ofFIG. 3, for example, are not limited, it is preferable that the layersare doped with the n-type impurity which enables efficient injection ofthe carriers from the n-type layer thereby improving the reliability ofthe device.

By making the first p side barrier layer and said second n side barriersubstantially equal in thickness, barrier layers are provided with theoutermost layers of the active layer being substantially symmetrical, sothat variations in the devices are suppressed thereby to improve theyield of production. This is believed to be the result of, throughdetailed mechanism is not known, the first p side barrier layer and thesecond n side barrier layer which function as the entrance for injectingthe carriers of the p-type layer and the n-type layer being configuredsymmetrically, which improves the symmetry of the layer structure of theactive layer and results in lower threshold current and stable lifetimeof the device.

In the fourth embodiment, the active layer has two or more well layersand a third barrier layer disposed between the well layers, whilethickness of the third barrier layer is smaller than the thickness ofthe first p side barrier layer and the second n side barrier layer. Thisconstitution makes it possible to improve the device characteristicsfurther. That is, the second n side barrier layer and the first p sidebarrier layer which are disposed at the outermost positions of theactive layer function as the entrance for injecting the carriers of thep-type layer and the n-type layer, respectively, and have largerthickness than the other barrier layers, so as to secure a space largerenough to hold a large amount of carriers, thus enabling stableoperation of the device even with large current. On the other hand,since the third barrier layer is sandwiched by the well layers, itsuffices to provide the layer so that the carriers are injected intoeach well layer and communication between the well layers is achieved,thus it is not necessary to form in large thickness unlike the outermostbarrier layers. In addition, with such a structure as the thick barrierlayers disposed as the outermost layers and thin barrier layers disposedin the middle of the active layer, the first p side barrier layer andthe second n side barrier layer which are thicker than the third barrierlayer function as strong barrier layers located on the opposite sideswith respect to the n-type layer and the p-type layer while the carriersfrom the n-type layer and the p-type layer are injected by means of theouter barrier layers, thereby accelerating the injection of the carrierinto each well layer and the light emission recombination. Making thethird barrier layer thinner than the outer barrier layers makes itpossible to keep the total thickness of the active layer small, thuscontributing to the decrease in the threshold current density and thevalue of Vf.

As will be understood from the foregoing description, the first tofourth embodiments have the following features.

In the first to fourth embodiments, injection of the carriers into theactive layer is accelerated because the barrier layers (the firstbarrier layer, the barrier layer B₁ and the first p-type barrier layer)which are located nearest to the p-type layer in the active layer do notsubstantially-contain the n-type impurity, thereby achieving the nitridesemiconductor device having excellent device lifetime and high outputpower. Moreover, by including the p-type impurity, injection of carrierand light emission recombination can be done efficiently even with alarge amount of carriers, thereby achieving the nitride semiconductordevice having excellent device lifetime and high output power. At thistime, when the barrier layer located nearest to the p-type layerincludes the p-type impurity, the layer is preferably grown undoped orso as not to substantially include r-type impurity. This is because, incase the n-type purity is included when the barrier layer locatednearest to the p-type layer includes the p-type impurity, carrierinjection from the p-type layer rends to be impeded leading to weakereffect of injecting a large amount of carriers efficiently, thusresulting in shorter device lifetime and lower output power.

Embodiment 5: Laser Device

The fifth embodiment of the nitride semiconductor device of the presentinvention is a laser device having at least such a structure as anactive layer is sandwiched by an n-type cladding layer and a p-typecladding layer in a p-type nitride semiconductor layer and an n-typenitride semiconductor layer. An optical guide layer which interposes theactive layer may also be provided between the cladding layer and theactive layer. In the forth embodiment of the present invention,waveguide structure may be applicable not only laser device but alsoedge emitting LED or super luminescent diode.

FIG. 1 is a sectional view showing an example of the nitridesemiconductor layer of the present invention. An active layer 107 madeof In_(x)Ga_(1-x)N (0≦x<1) is sandwiched by n-type Al_(y)Ga_(1-y)N(0≦y<1) layers 103 to 106 (every layer has different value of y) andp-type Al_(z)Ga_(1-z)N (0≦z<1) layers 108 to 111 (every layer hasdifferent value of z) on a GaN substrate 101, thus constituting theso-called double hetero structure.

The n-type cladding layer and the p-type cladding layer are made of anitride semiconductor which includes Al, and specificallyAl_(b)Ga_(1-b)N (0<b<1) is preferably used.

According to the present invention, there is no limitation to thecomposition of the optical guide layer which is required only to have asufficient energy band gap for forming the waveguide, whether made insingle film or multiple-film constitution. For example, refractive indexof the waveguide can be made higher by using GaN for wavelengths from370 to 470 and using multiple-film constitution of InGaN/GaN for longerwavelengths. Thus various nitride semiconductors such as InGaN, GaN andAlGaN can be used. The guide layer and the cladding layer may also bemade in super lattice multiple-film structure.

Now detailed structure of the nitride semiconductor laser shown in FIG.1 will be describe below. Formed on the substrate 101 via the bufferlayer 102 are the n-type contact layer 103 which is an n-type nitridesemiconductor layer, the crack preventing layer 104, the n-type claddinglayer 105 and the n-type optical guide layer 106. Layers other than then-type cladding layer 105 may be omitted depending on the device. Then-type nitride semiconductor layer is required to have a band gap widethan the active layer at least in a portion which makes contact with theactive layer, and therefore preferably has a composition which includesAl. The layers may also be made n-type either by growing while dopingwith an n-type impurity or by growing undoped.

The active layer 107 is formed on the n-type nitride semiconductorlayers 103 to 106. The construction of the active layer is as describedpreviously.

Formed on the last barrier layer are the p-type electron confinementlayer 108 as the p-type nitride semiconductor layer, a p-type opticalguide layer 109, a p-type cladding layer 110 and a p-type contact layer111. Layers other than the p-type cladding layer 110 may be omitteddepending on the device. The p-type nitride semiconductor layer isrequired to have a band gap wide than the active layer at least in aportion which makes contact with the active layer, and thereforepreferably has a composition which includes Al. The layers may be madep-type either by growing while doping with a p-type impurity or by usingthe diffusion of the p-type impurity from the other layer which adjoinsthereto.

The p-type electron confinement layer 108 made of a p-type nitridesemiconductor layer which includes Al in a proportion higher than in thep-type cladding layer 110, preferably having a composition ofAl_(x)Ga_(1-x)N (0.1<x<0.5). This layer is also heavily doped with ap-type impurity such as Mg preferably in a concentration from 5×10¹⁷/cm³to 1×10¹⁹/cm³. This makes the p-type electron confinement layer 108capable of effectively confining electrons within the active layer, thusdecreasing the threshold of the laser. The p-type electron confinementlayer 108 may be formed in a thin film of about 30 to 200 Å. Such a thinfilm can be grown at a temperature lower than the temperature forgrowing the p-type optical guide layer 109 and the p-type cladding layer110. By forming the p-type electron confinement layer 108 as describedabove, decomposition of the active layer which includes In can besuppressed compared to the case where the p-type optical guide layer 109and the like are formed directly on the active layer.

The p-type electron confinement layer 109 plays the role of supplyingthe p-type impurity through diffusion into the last barrier layer whichis grown undoped. These layers work in cooperation to protect the activelayer 107 from decomposition and improve the efficiency of injectingholes into the active layer 107. That is, by forming the undopedIn_(x2)Ga_(1-x2)N well layer (0≦x₂<1) as the last layer of the MQWactive layer to be thicker than the other barrier layers and growingthereon the thin film made of p-type Al_(x)Ga_(1-x)N (0.1<x<0.5) heavilydoped with the p-type impurity such as Mg at a Low temperature, it ismade possible to protect the active layer 107 from decomposition andimprove the efficiency of injecting holes into the active layer throughdiffusion of the p-type impurity such as Mg from the p-typeAl_(x)Ga_(1-x)N layer into the undoped In_(x2)Ga_(1-x2)N layer.

Among the p-type nitride semiconductor layers, ridge stripe is formed upto midway in the p-type optical guide layer, and the semiconductor laseris made by forming protective films 161, 162, a p-type electrode 120, ann-type electrode 121, a p-type pad electrode 122 and an n-type padelectrode.

(Eelectron Confinement Layer: First p-type Nitride Semiconductor Layer)

According to the present invention, the first p-type nitridesemiconductor layer is preferably provided as the p-type nitridesemiconductor layer particularly in the laser device. The first p-typenitride semiconductor layer is made of a nitride semiconductor whichincludes Al, and specifically Al_(a)Ga_(1-a)N (0<a<1) is preferablyused. The proportion γ of Al is determined so the layer functions as theelectron confinement layer when used in a laser device which requires itthat a sufficiently larger band gap energy is provided (offset) thanthat of the active layer, and is set at least in a range of 0.1≦γ<1, andpreferably in a range of 0.2≦γ<0.5. This is because the value of y lessthan 0.1 makes it unable to fully function as the electron confinementlayer in the laser device. When the value of γ is 0.2 or greater,electrons (carrier) can be sufficiently confined and carrier overflowcan be suppressed. When the value of γ is not larger than 0.5, the layercan be grown while minimizing the occurrence of cracks. Further, thevalue of γ is preferably set to 0.35 or less which allows it to achievegood crystallinity. At this time, proportion of Al in the composition isset higher than in the p-type cladding layer, because confinement of thecarrier requires a nitride semiconductor having higher proportion of Althan in the cladding layer which confines light. The first p-typenitride semiconductor layer can be used in the nitride semiconductordevice of the present invention and, particularly in the case of a laserdevice which is driven with a large current while injecting a largeamount of carriers into the active layer, the carriers can be confinedmore efficiently than in the case where the first p-type nitridesemiconductor layer is not provided, making applicable to high outputLEDs as well as the laser device.

Thickness of the first p-type nitride semiconductor layer of the presentinvention should be not larger than 1000 Å, preferably 400 Å or smaller.This is because the nitride semiconductor which includes Al has bulkresistance higher than other types of nitride semiconductor (without Alcontent), and therefore makes a layer of extremely high resistance whenformed with a thickness greater than 1000 Å, thus resulting in asignificant increase in the forward voltage Vf. When the thickness isnot larger than 400 Å, the increase in Vf can be kept at a low level.More preferably the thickness is 200 Å or less which makes it possibleto suppress the increase in Vf even lower. Lower limit of the thicknessof the first p-type nitride semiconductor layer is 10 Å or larger,preferably 50 Å or larger, which allows the electron confinement layerto function satisfactorily.

In the laser device, the first p-type nitride semiconductor layer isprovided between the active layer and the cladding layer so as tofunction as an electron confinement layer or, when a guide layer isadditionally provided, provided between the guide layer and the activelayer. At this time, the distance between the active layer and the firstp-type nitride semiconductor layer is not larger than 1000 Å whichenables it to function as the carrier confinement layer, and preferably500 Å or smaller for better carrier confinement. This is because thefirst p-type nitride semiconductor layer has better effect of carrierconfinement when nearer to the active layer. Moreover, since it is notnecessary to have another layer between the active layer and the firstp-type nitride semiconductor layer in most cases in the laser device, itis usually most preferable to provide the first p-type nitridesemiconductor layer so as to make contact with the active layer. At thistime, in case the crystallinity is adversely affected by providing thelayer located nearest to the p-type nitride semiconductor layer in theactive layer of the quantum well structure and the first p-type nitridesemiconductor layer in contact with each other, a buffer layer may beprovided between both layers when growing the crystal, in order to avoidthe adverse effect. For example, a buffer layer made of GaN maybeprovided between The outermost p side layer of the active layer and thefirst p-type nitride semiconductor layer such as InGaN or AlGaN, or abuffer layer made of nitride semiconductor which includes Al in lowerproportion than that in the first p-type nitride semiconductor layer maybe provided.

The positional relationship between the first p-type nitridesemiconductor layer and the active layer, particularly the distance fromthe well layer is an important factor that determines the thresholdcurrent density and lifetime of the laser device. Specifically, thethreshold current density can be made lower as the first p-type nitridesemiconductor layer is located nearer to the active layer, but this alsoleads to shorter lifetime of the device. This is supposedly because, asdescribed above, the first p-type nitride semiconductor layer has aresistance far higher than that of the other layer and generates greaterheat during operation of the device thus reaching a higher temperaturein the device. This may be affecting the active layer and the welllayers which are vulnerable to heat, thus causing the device lifetimesignificantly. On the other hand, as described above, the first p-typenitride semiconductor layer which bears the function of carrierconfinement can perform more effective carrier confinement function whenlocated nearer to the active layer, with the effect diminishing as thedistance from the active layer increases.

Therefore, keep the lifetime of the device from decreasing, distance dBbetween the well layer 1 and the first p-type nitride semiconductorlayer 28 should be 100 Å or larger, preferably 120 Å or larger, and mostpreferably 140 Å or larger as shown in FIG. 8A, When the distance dBbetween the well layer and the first p-type nitride semiconductor layeris shorter than 100 Å, the device lifetime decreases sharply. When thedistance is not less than 120 Å, the device lifetime can be increasedsignificantly. When the distance is larger than 150 Å, the devicelifetime increases further, while the threshold current density beginsto gradually increase. When the distance is larger than 200 Å, shethreshold current density shows clear tendency to increase and, beyond400 Å, the threshold current density increases sharply. Accordingly,upper limit of the distance is set within 400 Å, preferably within 200Å. The mechanism behind this is believed to be such that the efficiencyof carrier confinement decreases as the first p-type nitridesemiconductor layer departs from the well layer, which becomes the majorcause and increase in the threshold current density, resulting in thedecrease in the efficiency of light emission.

The well layer used as the datum for distance is the well layer 1 bwhich is disposed on the side of the n-type layer while adjoining thebarrier layer 2 c provided nearest to the p-type layer 13 in the activelayer. In case the first p-type nitride semiconductor layer is providedin contact with the active layer of quantum well structure, either theconstitution shown in FIG. 8A wherein the first p-type nitridesemiconductor layer 28 is provided in contact with the barrier layer 2 clocated nearest to the p-type layer side or the constitution shown inFIG. 8B wherein the well layer 4 is provided between the first p-typenitride semiconductor layer 28 and the barrier layer 2 c located nearestto the p-type layer 13 side may be employed. In the constitution wherethe well layer 4 is provided between the first p-type nitridesemiconductor layer 28 and the barrier layer 2 c located nearest to thep-type layer 13 side, the well layer 4 becomes too near to the p-typelayer 13, so that most of the carriers injected from the p-type layerpass through the well layer 4 without experiencing light emissionrecombination in the well layer 4, thus the function of the well layeris not performed. In case the barrier layer 2 c located nearest to thep-type layer side includes the p-type impurity, the carriers can beinjected better into the well layers 1 a, 1 b which are located nearerto the n-type layer side than the barrier layer 2 c, while the carrierspass through the well layer 4 which is located nearer to the p-typelayer side than the barrier layer 2 c, thus further losing thecontribution to light emission recombination and the function of thewell layer is rapidly lost. For this reason, between the well layer 4and the first p-type nitride semiconductor layer shown in FIG. 8B, thereis no change in the characteristic due to the distance as describedabove, and the distance d_(B) from the well layer becomes the distancefrom the well layer located at a position nearer to the n-type layerside than the barrier layer nearest to the p-type layer regardless ofthe well layer 1 c located at a position nearer to the p-type layer sidethan the barrier layer nearest to the p-type layer. And even when a welllayer which becomes the layer nearest to the p-type layer is provided inthe active layer, change in the characteristics due to the distanced_(B) as described above is observed. Such a well layer which is locatedat a position nearer to the p-type layer side than the barrier layernearest to the p-type layer does not fully function as a well layer, andalso cause the device characteristics such as lifetime to deterioratecompared to a case where the well layer is not provided. Therefore, sucha constitution is preferably employed as a layer located nearest to thep-type layer in the active layer is used as the barrier layer withoutproviding the well layer, as shown in FIG. 8A rather than that shown inFIG. 8B.

In case the first p-type nitride semiconductor layer 28 is provided incontact with the barrier layer 2 c located nearest to the p-type layerside, the barrier layer 2 c (barrier layer nearest to the p-type layerside) may be provided between the well layer and the first p-typenitride semiconductor layer, so as to determine the distance d_(B) bythe thickness of this barrier layer. Therefore, thickness of the barrierlayers (the first barrier layer, the barrier layer B_(L) and the firstp-type barrier layer) which are located nearest to the p-type layerbecomes an important factor that determines the characteristics of thenitride semiconductor device. In addition, since the increase in thethreshold current density of a laser device is caused mainly by theconfinement of the carrier described above, the relationship between theactive layer and the first p-type nitride semiconductor layer applieshere again.

The first p-type nitride semiconductor layer of the present invention isusually doped with a p-type impurity, and is doped with a highconcentration in order to improve the carrier mobility in the case of alaser device or a high power LED which are driven with a large current.Specifically, doping concentration is 5×10¹⁶/cm³ or higher, andpreferably 1×10¹⁸/cm³ or higher. In the case of a device driven with alarge current, doping concentration is 1×10¹⁸/cm³ or higher, andpreferably 1×10¹⁹/cm³ or higher. Concentration of the p-type impurity isnot limited, but is preferably 1×10²¹/cm³ or higher. However, when theconcentration of the p-type impurity is too high, bulk resistance tendsto increase resulting in higher value of Vf. In order to avoid thisproblem, minimum necessary concentration of p-type impurity to securethe required level of carrier mobility is preferably provided. In case ap-type impurity which has a strong tendency to diffuse such as Mg isused, the first p-type nitride semiconductor layer may be grown withoutdoping and then doped by the diffusion of impurity from an adjacentlayer, for example, an optical guide layer. Or, alternatively, in casethe first p-type nitride semiconductor layer is grown without dopingwhile a layer doped with the p-type impurity exists other than theadjacent layer or the p-type impurity diffused region, and diffusion ofimpurity does not occur in the first p-type nitride semiconductor layer,then the layer may be provided undoped when the thickness allows thecarrier to tunnel therethrough.

In addition to the above, in the laser device of the present invention,in case a p-type optical guide layer is provided in contact with thefirst p-type nitride semiconductor layer, a good optical guide layer isobtained by doping with a p-type impurity by the diffusion from thefirst p-type nitride semiconductor layer. Since the p-type impurity inthe guide layer scatters light in the optical waveguide, inclusion ofimpurity with as low concentration as possible within such a range thatensures electrical conductivity is preferable for improving the devicecharacteristics. However, doping with p-type impurity when growing thep-type optical guide layer has a problem of difficulty to control thedoping with impurity in a low concentration region where the loss oflight can be kept low. This is because, while the nitride semiconductordevice generally has such a structure as an n-type layer, an activelayer and a p-type layer are stacked in this order, it is necessary toprevent In from decomposing during the subsequent process of growing thelayers because of InGaN included in the active layer, when thisstructure is grown. Although a method of growing the p-type layer at alow temperature in a range from 700 to 900° C. is commonly employed, itbecomes difficult to control the doping impurity concentration becauseof the low temperature. Also Mg is commonly used as the p-type impurity,it is relatively difficult to control the doping impurity concentration,and variations may be caused in the device characteristics when dopingthe impurity in the low concentration region when growing.

Therefore, it is preferable that the first p-type nitride semiconductorlayer plays the role of the impurity supplying layer by doping the firstp-type nitride semiconductor layer with the impurity in a highconcentration during growth thereof, while giving consideration to thediffusion of the p-type impurity into the optical guide layer. Furtherin the embodiments described above, the barrier layers (the firstbarrier layer, the barrier layer B and the first p-type barrier layer)which are disposed in contact with the first p-type nitridesemiconductor layer can also be caused to play the role of the impuritysupplying layer by doping with the p-type impurity, too.

In the laser device of the present invention, as shown in theembodiments, an insulation film which becomes a buried layer is formedin the side face of a ridge after forming the ridge. For the buriedlayer, a material other than SiO₂, preferably an oxide which includes atleast one kind of element selected from among a group consisting of Ti,V, Zr, Nb, Hf and Ta, or at least one of SiN, BN, SiC and AlN is usedand, among these, it is particularly preferable to use an oxide of Zr orHf, or BN, SiC. As the buried layer, half-insulating i-type nitridesemiconductor; a nitride semiconductor of conductivity type opposite tothat of the ridge (n-type in the case of the embodiment); a nitridesemiconductor including Al such as AlGaN (when current pinching layer isneeded) can be used. Alternatively, in order to form light wave guide,the buried layer is made to absorb much more light than the ridge partby employing nitride semiconductor layer including In (such as InGaN) asthe buried layer. Such a structure may also be employed as ions ofelements such as B or Al are injected without forming a ridge by etchingor the like, while forming a region without the ion being injected whichis the current flowing region. At this time, nitride semiconductorrepresented as In_(x)Al_(1-y)Ga_(1-x-y)N, 0≦x≦1, 0≦y≦1, x+y=1).

The first p-type nitride semiconductor layer of the present inventionperforms carrier confinement as described above, and the first p-typenitride semiconductor layer can be applied as a cladding layer even whenonly the classing layer for carrier confinement is provided withoutneeding an optical confinement cladding layer in a light emittingdevice, as shown in an embodiment.

Moreover, while the first p-type nitride semiconductor layer has such aconstitution as band offset is provided between the active layer inorder to confine electrons in the active layer, namely making the bandgap energy higher than that in the active layer with a voltage barrierbetween both layers, it is preferable to provide band gap energy higherthan that in the guide layer in a laser device of SCH structure. In casecladding layer comprising two or more layers of different band gapenergies is provided, the first p-type nitride semiconductor layer isprovided in the cladding layer on the active layer side, preferably withband gap energy higher than that in the other layers. Specifically, sucha structure may be employed as the first p-type nitride semiconductorlayer having a high band gap energy is used as the first cladding layerand a second cladding layer having lower band gap energy is disposedmore distantly from the active layer than the first cladding layer, suchas the structure of the first embodiment minus the guide layer.

As described above, when the first barrier layer (barrier layer nearestto the p side) is considered based on the first p-type nitridesemiconductor layer, with regards to the carrier confinement in theactive layer being determined by the first p-type nitride semiconductorlayer which plays the role of carrier confinement in the band structureshown in FIGS. 8A and 8B, the active layer may be the region from theposit ion of making contact with the first p-type nitride semiconductorlayer in the present invention. That is, in case the barrier layernearest to the p side has a function different from that of the otherbarrier layer, because of the close relationship with the first p-typenitride semiconductor layer which plays the role of carrier confinementas described above, the region from the interface in contact with thefirst p-type nitride semiconductor layer can be regarded as the activelayer. Thus in consideration of the fact that the region from theposition of making contact with the first p-type nitride semiconductorlayer is the active layer, the effect of the first barrier layer, thebarrier layer nearest to the p side of the present invention can beachieved even when some layer, such as the well layer 4 shown in FIG.8B, for example, is interposed between the first barrier layer and thefirst p-type nitride semiconductor layer. Specifically, in addition tothe embodiment shown in FIG. 8B, a layer having an intermediate band gapenergy may be interposed between the first barrier layer and the firstp-type nitride semiconductor layer. Moreover, as described above, sincethe first barrier layer, the barrier layer nearest to the p side have afunction significantly different from the other barrier layer, forexample the barrier layer sandwiched by the well layers, the layer mayhave different composition and band gap energy.

By setting the ridge width in a range from 1 μm to 3 μm, preferably from1.5 μm to 2 μm, light source for an optical disk system having good spotshape and beam configuration can be obtained. The laser device of thepresent invention is now limited to the refractive index guiding typewaveguide of ridge structure, and gain guiding type may be employed.Also the BH structure where ridge side face is buried by regrowth, astructure having a ridge buried by regrowth, or a structure havingcurrent pinching layer may be employed, and the active layer describedabove is effective for any laser device structure.

Embodiment 6

The sixth embodiment is the nitride semiconductor device described abovehaving such a laser device structure as the active layer of quantum wellstructure is sandwiched between an upper cladding layer made of anitride semiconductor including Al and a lower cladding layer made of anitride semiconductor including Al, wherein the upper cladding layer andthe lower cladding layer include Al in mean proportion x being 0<x≦0.05.This constitution makes it possible to loosen the confinement in theoptical waveguide sandwiched by the upper cladding layer and the lowercladding layer by controlling the proportion of Al in the claddinglayers to 0.05 or less, and suppressing the self-excited oscillation bycontrolling the thickness ratio of the barrier layer and the well layerof the active layer, thereby improving the output characteristic and thelifetime of the device. The laser device made in this constitution iscapable of continuous oscillation with an output power of 5 to 100 mW,thus making a laser device having characteristics suitable for thereading and writing light sources of an optical disk system whileachieving a longer lifetime of the device. Preferably, as shown in FIGS.3, 4, 6, 7, an optical guide layer is provided between the upper and thelower cladding layers so as to difference in the refractive index in thecladding layers and spread the distribution of light, which causes thelight to be distributed widely in the guide layer thereby decreasing theloss due to leaking light.

When the well layer has thickness of 40 Å or larger and the thicknessratio R_(t) is in a range from ⅓ to 1 in the active layer, the devicecharacteristics can be improved in the first to fourth embodiments andin the fifth embodiment. Although the mechanism which improves thedevice characteristics is not known, such a structure has been used inthe prior art as the probability of light emission recombination tooccur in the well layer is increased by providing a barrier thicknesssufficiently thicker than the well layer. In contrast, in the activelayer described above, the well layer is made as thick as 40 Å or largerand the barrier layer is made thinner compared to the well layer. Thusthe thick well layer provides greater region for light emissionrecombination to occur, and the thin barrier layer provided between thewell layers allows the carriers to be injected evenly into the welllayers, thus increasing the probability of light emission recombinationto occur. In the case of a high output device, while a large amount ofcarriers are injected into the well layer because of driving with alarge current, the thick well layer provides greater region for lightemission recombination to occur, and the thin barrier layer tends toenable uniform injection into the well layers passing through thebarrier layer.

When the well layer has thickness of 40 Å or larger and the thicknessratio R_(t) (R_(t)=[thickness of well layer]/[thickness of barrierlayer]) is in a range from ⅓ to 1 in the active layer, the laser devicehaving excellent characteristics for the light source of an optical disksystem is obtained. This is because making the well layer 40 Å orthicker results in the long lifetime of the device as shown in FIG. 12,and setting in the range described above keeps the value of RIN(relative intensity of noise) low. More preferably, thickness of thewell layer is set to 50 Å or larger which makes it possible to increasethe device lifetime further. When the value of R_(t) is 1 or greater,although the value of RIN increases, device lifetime becomes longer anda laser device having a large output power can be made, thus allowingapplications other than the optical disk system, Based on the foregoingdiscussion, thickness of the barrier layer is preferably 40 Å or largerbecause this provides the laser device having long lifetime as shown inFIG. 13.

Embodiment 7: Number of Well Layers

In the first to sixth embodiments described above, the number of welllayers provided in the active layer is set in a range from 1 to 3 anitride semiconductor device having good device characteristicsperformed even under operation with a large current can be obtained. Inthe prior art, while the number of well layers in the active layer hasbeen set in a range from about 4 to 6, the large number of the welllayers increases the probability of the carrier recombination but thetotal thickness of the active layer including the barrier layer becomeslarger and tends to increase the value of Vf. Also it has been foundthat increasing the number of well layers does not cause a correspondingincrease in the probability of the carrier recombination. In the case ofLD which is driven with a large current and high current density, inparticular, this tendency is observed more markedly. In the case of LD,for example, when the number of well layers is changed in multiplequantum well structure, threshold current tends to decrease as thenumber of well layers decreases, and shows sharp decrease while thenumber of well layers decreases from 6 to 4, gradual decrease as thenumber decreases from 4 to 3, reaches a minimum value the number of welllayers is 2 or 3. When the number of well layer is 1, namely in the caseof single quantum well structure, threshold current becomes equal to ora little higher than that of the case where the number of well layers is2 or 3. Similar tendency is seen also in an LED of high output power.

The accompanying drawings will be described below. FIGS. 2, 3 areschematic sectional views of an embodiment of the present invention,particularly showing a structure of a laser device where an active layer12 is sandwiched by an n-type layer 11 and a p-type layer 13. FIG. 2shows a structure where the active layer 12 is sandwiched by an uppercladding layer 30 and a lower cladding layer 25, a first p-type nitridesemiconductor layer 28 which is an electron confinement layer isprovided between the active layer 12 and the upper cladding layer 30,with the quantum well structure of the active layer 12 being made bystacking pairs of barrier layer 2 a/well layer 1 a repetitively andproviding the barrier layer 2 c at the end. Difference of the structureshown in FIG. 3 from that of FIG. 2 is that upper and lower opticalguide layers 29, 26 are provided between the upper and lower claddinglayers 30, 25 and the active layer 12. FIGS. 4 to 8 and FIG. 10 showstacked structure 20 of or around the active layer 12 and an energy bandgap 21 provided below and corresponding to the stacked structure 20.FIGS. 4, 6 show the quantum well structure of the active layer 12 havingan asymmetrical structure with regard to the film thickness. FIGS. 5, 7show, on the contrary, a symmetrical structure. The number of the welllayers in the active layer is 3 in FIGS. 4, 5 and 2 in FIGS. 6, 7. FIG.5 shows a structure without optical guide layer, while FIGS. 4, 7, 8show a structure having optical guide layer. FIG. 8 shows a structurewhere the active layer 12 and the p-type layer 13 are stacked, showingthe relationship between the first p-type nitride semiconductor layer 28and the active layer in the p-type layer 13, the barrier layer 2 cprovided at a position nearest to the p-type layer side and the welllayer 1 b located nearer to the n-type layer than the barrier layer 2 c.

Embodiment 8

The eighth embodiment of the present invention provides a laser devicehaving fast response characteristics and RIN suitable for optical disksystems such as DVD and CD. Specifically, the active layer of quantumlayer structure comprises a first barrier layer (barrier layer disposednearest to the p side) and a second barrier layer the ratio R_(t) ofthickness between the well layer and the second barrier layer is in arange of 0.5≦R_(t)≦3. The first barrier layer (barrier layer disposednearest to the p side) and the second barrier layer are similar to thoseof the embodiments described above. With this thickness ratio, it isimportant that the second barrier layer be the barrier layer sandwichedby he well layers in the MQW, namely at the distance between the welllayers. As described above, since the barrier layer located nearest tothe p side and the other barrier layers have different functions, forthe barrier layers which affect the response characteristics and RIN,the barrier layers other than the first barrier layer (barrier layerdisposed nearest to the p side) are important. Particularly in the MQW,ratio of the thickness of the barrier layer sandwiched by the welllayers and the thickness of the well layer have a great influence on thecharacteristics described above. When the thickness ratio R_(t) iswithin the range described above, a good laser device suited to thelight source of an optical disk system is obtained. When the ratio isbelow 0.5, thickness of the barrier layer becomes too large compared tothe well layer leading to a degradation in the response characteristic.When the ratio exceeds 3, RIN is adversely affected thus making a lightsource having significant noise when high frequency is superimposed. Theratio is preferably set in a range of 0.8≦R_(t)≦2 which leads to a laserdevice excellent in the characteristics described above. At this time,film thickness d_(w) of the well layer is preferably in a range of 40Å≦d_(w)≦100 Å. This is because better device lifetime can be achieved asthe well layer becomes thicker in the embodiments described above aswill be seen from FIG. 12. When the film thickness is larger than 100 Å,degradation of the response characteristics and RIN becomes moresignificant thus making the device not suitable for the light source ofan optical disk system. The thickness is preferably in a range of 60Åd_(w)<80 Å. This is because, while thicker well layer leads to slowerrate of deterioration which is another criterion for evaluating thelifetime of the device, the rate of deterioration shows a sharp decreasewhen thickness of the well layer is increased in a range from 40 Å to 80Å, and shows a gradual decrease when the thickness exceeds 80 Å. Thefilm thickness d_(b) of the second barrier layer is set to 40 Å orlarger for the consideration to the relation between the film thicknessand the device lifetime shown in FIG. 13, and a laser device ofexcellent device lifetime is obtained in this range of thickness.

This embodiment is preferably combined with the first to seventhembodiment. The second barrier layer is at least the barrier layerprovided in the active layer in the embodiment shown in FIGS. 6, 7, andis applied to some of the barrier layers other than the barrier layer 2c provided at a position nearest to the p-type layer side (first barrierlayer). Preferably it is applied to the barrier layer 2 b sandwiched bythe well layers, and most preferably applied to all the barrier layersother than the barrier layer nearest to the p side, because thisimproves the characteristics described above.

EXAMPLE 1

Now a laser device made of the nitride semiconductor device having thelaser device structure as shown in FIG. 8 will be described below as anexample.

While the substrate 101 is preferably made of GaN, a substrate of amaterial different from the nitride semiconductor may also be used. Thesubstrate of different material may be made of an insulating substancesuch as sapphire or spinel (MgAl₂O₄) having principal plane in the Cplane, R plane or A plane, or SiC (6H, 4H, 3C), ZnS, ZnO, GaAs, Si or amaterial other than nitride semiconductor which has been known to becapable of growing nitride semiconductor such as an oxide that undergoeslattice matching with the nitride semiconductor. Preferred material formaking the substrate of different material is sapphire or spinel. Thesubstrate of different material may be an off-angle one, in which caseit has preferably stepwise off-angle construction for this allows baselayer of gallium nitride to grow with good crystallinity. When asubstrate of different material is used, devices maybe formed in theform of single substrate of nitride semiconductor by removing thesubstrate of different material by polishing or other method aftergrowing the nitride semiconductor which makes the base layer on thesubstrate of different material before forming the device structure, oralternatively the substrate of different material may be removed afterforming the devices.

In case the substrate of different material is used, the nitridesemiconductor can be grown satisfactorily when the devices are formedvia the base layer made of a buffer layer (low-temperature grown layer)and nitride semiconductor (preferably GaN). For the base layer (growthsubstrate) provided on the substrate of different material, nitridesemiconductor grown by ELOG (Epitaxially Laterally Overgrowth) may alsobe used which allows it to obtain a growth substrate of goodcrystallinity. Specific examples of ELOG growth layer include one wherea nitride semiconductor layer is grown on a substrate of differentmaterial, whereon a mask region is formed by, for example, providing aprotective film which makes it difficult to grow the nitridesemiconductor, and a non-mask region is formed in stripes for growingthe nitride semiconductor, while the nitride semiconductor is grownthrough the non-mask region so that the growth proceeds in the lateraldirection as well as in the direction of thickness, thereby forming alayer with the nitride semiconductor growing also in the mask region. Inother aspect, the layer may also be formed by making an aperture in thenitride semiconductor grown on the substrate of different material andmaking lateral growth from the side face of the aperture.

(Substrate 101)

For the substrate, a nitride semiconductor, GaN in this example, isgrown into a thick film (100 μm) on a substrate made of a differentmaterial. With the substrate of the different material being removed, anitride semiconductor substrate made of GaN with a thickness of 80 μm isused. Detailed process of forming the substrate is as follows. Asubstrate of different material made of sapphire with the principalplane lying in the C plane having diameter of 2 inches is set in a MOVPEreaction vessel, of which temperature is set to 500° C., and a bufferlayer made of GaN is formed to a thickness of 200 Å by using trimethylgallium (TMG) and ammonia (NH₃). With the temperature raised, a film ofundoped GaN 1.5 μm is grown as a base layer. Then with a plurality ofstriped masks formed on the base layer surface, a nitride semiconductor,GaN in this example, is selectively grown through apertures (windows) ofthe mask. The nitride semiconductor layer formed by a growing processinvolving lateral growth (ELOG) is further grown to become thicker. Thenthe nitride semiconductor substrate is obtained by removing thesubstrate of different material, the buffer layer and the base layer. Atthis time, the mask used in the selective growth is made of SiO₂havingmask width of 15 μm and aperture (opening) width of 5 μm.

(Buffer Layer 102)

With temperature set to 1050° C., a buffer layer 102 made ofAl_(0.25)Ga_(0.95)N is formed to a thickness of 4 μm on the nitridesemiconductor substrate, which has been formed as described above, byusing TMG (trimethyl gallium), TMA (trimethyl aluminum) and ammonia.This layer functions as a buffer layer between the n-type contact layermade of AlGaN and the nitride semiconductor substrate., Then layerswhich constitute the device structure are formed on the base layer madeof nitride semiconductor.

Specifically, in the case of a substrate made of GaN formed by lateralgrowth, generation of pits can be decreased by using the buffer layer102 made of nitride semiconductor Al_(a)Ga_(1-a)N (0<a≦1) which hasthermal expansion coefficient smaller than that of GaN. Preferably thebuffer layer is formed on the laterally grown layer of nitridesemiconductor GaN. When the proportion of Al in the buffer layer 102 isin a range of 0<a<0.3, the buffer layer can be formed with goodcrystallinity. The buffer layer may be formed as an n-type contactlayer, or the buffer layer 102 and the n-type contact layer 104 formedthereon may be caused to perform buffering effect by forming the n-typecontact layer of the sane composition as that of the buffer layer afterforming the buffer layer 102. That is, as the buffer layer 102 of atleast one layer is provided between the nitride semiconductor substratewhich employed, lateral growth or a laterally grown layer formed thereonand the device structure, or between the active layer in the devicestructure and the laterally grown layer (substrate) or the laterallygrown layer (substrate) formed thereon, or more preferably between thelower cladding layer provided on the substrate side of the devicestructure and the laterally grown layer (substrate), generation of pitscan be decreased and the device characteristics ca be improved. Thebuffer layer is capable of improving the crystallinity when forming theactive layer, particularly the thick nitride semiconductor layer whichincludes In according to the present invention, it is preferable toprovide the buffer layer.

(n-type Contact Layer 103)

The n-type contact layer 103 made of Al_(0.05)Ga_(0.95)N doped with Siis formed to a thickness of 4 μm at a temperature of 1050° C. on thebuffer layer 102, which has been formed as described above, by usingTMG, TMA, ammonia, and silane gas used as an impurity gas.

(Crack Preventing Layer 104)

Then a crack preventing layer 104 made of In_(0.05)Ga_(0.95)N is formedto a thickness of 0.15 μm at a temperature of 800° C. by using TMG, TMI(trimethyl indium), and ammonia. The crack preventing layer may beomitted.

(n-type Cladding Layer 105)

After growing a layer A made of undoped Al_(0.05)Ga_(0.95)N to athickness of 25 Å is grown at a temperature of 1050° C. by using TMA,TMG and ammonia as the stock material gas, supply of TMA is stopped andsilane gas is used as the impurity gas, and a layer B made of GaN dopedwith Si in concentration of 5×10¹⁸/cm³ is formed to a thickness of 25 Å.This operation is repeated 200 times to stack the layer A and the layerB thereby to form the n-type cladding layer 106 made in multi-layeredfilm (super lattice structure) having a total thickness of 1 μm. At thistime, a difference in the refractive index which is sufficient for thecladding layer to function can be provided when the proportion of Al ofthe undoped AlGaN is in a range from 0.05 to 0.3.

(n-type Optical Guide Layer 106)

Then at a similar temperature, an n-type optical guide layer 106 made ofundoped GaN is formed to a thickness of 0.15 μm by using TMG and ammoniaas the stock material gas. The n-type optical guide layer 107 may alsobe doped with an n-type impurity.

(Active Layer 107)

Then by setting the temperature to 800° C., a barrier layer (B) made ofIn_(0.05)Ga_(0.95)N doped with Si in a concentration of 5×10^(18/cm) ³is formed to a thickness of 140 Å by using TMI (trimethyl indium), TMGand ammonia as the stock material gas and silane gas as the impuritygas. Then the supply of silane gas is stopped and a well layer (W) madeof undoped In_(0.1)Ga_(0.9)N is formed to a thickness of 25 Å, whilestacking the barrier layer (B) and the well layer (W) in the order of(B)/(W)/(B)/(W). Last, top barrier layer made of In_(0.05)Ga_(0.95)N isformed to a thickness of 140 Å by using TMI (trimethyl indium), TMG andammonia as the stock material gas. The active layer 107 becomes multiplequantum well structure (MQW) having total thickness of 470 Å.

(p-type Electron Confinement Layer 108: First p-type NitrideSemiconductor Layer)

Then at a similar temperature, a p-type electron confinement layer 108made of Al_(0.3)Ga_(0.7)N doped with Mg in a concentration of 1×10¹⁹/cm³is formed to a thickness of 100 Å by using TMA, TMG and ammonia as thestock material gas and Cp₂Mg (cyclopentadienyl magnesium) as theimpurity gas. This layer may not be provided, though this would functionas an electron confinement layer and help decrease the threshold whenprovided. In this case, the p-type impurity Mg diffuses from the p-typeelectron confinement layer 108 into the top barrier layer which isadjacent thereto so that the top barrier layer becomes doped with Mg ofabout 5 to 10×10¹⁶/cm³.

(p-type Optical Guide Layer 109)

Then by setting the temperature to 1050° C., a p-type optical guidelayer 109 made of undoped GaN is formed to a thickness of 0.15 μm byusing TMG and ammonia as the stock material gas. While the p-typeoptical guide layer 109 is grown as an undoped layer, diffusion of Mgfrom the adjacent layers such as the p-type electron confinement layer108 and the p-type cladding layer 109 increases the Mg concentration to5×10¹⁶/cm³ and turns the layer to p-type. Alternatively, this layer maybe intentionally doped with Mg while growing.

(p-type Cladding Layer 110)

Then a layer of undoped Al_(0.05)Ga_(0.95)N is formed to a thickness of25 Å at 1050° C., then supply of TMA is stopped and a layer of Mg-dopedGaN is formed to a thickness of 25 Å by using Cp₂Mg. This operation isrepeated 90 times to form the p-type cladding layer 110 constituted fromsuper lattice structure of total thickness of 0.45 μm. When the p-typecladding layer is formed in super lattice structure consisting ofnitride semiconductor layers of different band gap energy levels with atleast one nitride semiconductor layer including Al being stacked one onanother, crystallinity tends to be improved by doping one of the layersmore heavily than the other, in the so-called modulated doping. In thepresent invention, however, both layers may be doped similarly. Thecladding layer 110 is made of nitride semiconductor which includes Al,preferably in super lattice structure which includes Al_(X)Ga_(1-X)N(0<X<1), more preferably super lattice structure consisting of GaN andAlGaN stacked one on another. Since the p-type cladding layer 110 formedin the super lattice structure makes it possible to increase theproportion of Al in the entire cladding layer, refractive index of thecladding layer can be decreased. Also because the band gap energyincreases, it is very effective in reducing the threshold value.Moreover, since pits generated in the cladding layer can be reduced bythe super lattice structure compared to a case without super latticestructure, occurrence of short-circuiting is also reduced.

(p-type Contact Layer 111)

Last, at a temperature of 1050° C., a p-type contact layer 111 made ofp-type GaN doped with Mg in a concentration of 1×10²⁰/cm³ is formed to athickness of 150 Å on the p-type cladding layer 110. The p-type contactlayer 111 may be formed from p-type In_(x)Al_(y)Ga_(1-x-y)N (0≦X, 0≦Y,X+Y≦1), and preferably from Mg-doped GaN which achieves the best ohmiccontact with the p-type electrode 120. Since the contact layer 111 isthe layer where the electrode is to be formed, it is desirable to have ahigh carrier concentration of 1×10¹⁷/cm³ or higher. When theconcentration is lower than 1×10¹⁷/cm³, it becomes difficult to achievesatisfactory ohmic contact with the electrode. Forming the contact layerin a composition of GaN makes it easier to achieve satisfactory ohmiccontact with the electrode. After the reaction has finished, the waferis annealed in nitrogen atmosphere at 700° C. in the reaction vesselthereby to further decrease the electrical resistance of the p-typelayer.

After forming the nitride semiconductor layers one on another asdescribed above, the wafer is taken out of the reaction vessel. Then aprotective film of SiO₂ is formed on the surface of the top-most p-typecontact layer, and the surface of the n-type contact layer 103 whereonthe n-type electrode is to be formed is exposed as shown in FIG. 1 byetching with SiCl₄ gas in the RIE (reactive ion etching) process. Forthe purpose of deep etching of the nitride semiconductor, SiO₂ is bestsuited as the protective film.

Then ridge stripe is formed as the striped waveguide region describedabove. First, a first protective film 161 having thickness of 0.5 μm isformed from Si oxide (mainly SiO₂) over substantially the entire surfaceof the top-most p-type contact layer (upper contact layer) by means of aPDP apparatus. Then the first protective film 161 is patterned withstripe width of 1.6 μm with a mask of a predetermined configurationbeing placed thereon by means of photolithography process and the RIE(reactive ion etching) apparatus which employs CF₄ gas. At this time,height of the ridge stripe (depth of etching) is set so that thicknessof the p-type optical guide layer 109 becomes 0.1 μm by partiallyetching the p-type contact layer 111, the p-type cladding layer 109 andthe p-type optical guide layer 110.

After forming the ridge stripe, a second protective layer 162 made of Zroxide (mainly ZrO₂) is formed on the first protective layer 161 to athickness of 0.5 μm continuously over the first protective layer 161 andthe p-type optical guide layer 109 which has been exposed by etching.

After forming the second protective film 162, the wafer is subjected toheat treatment at 600° C. When the second protective film is formed froma material other than SiO₇, it is preferable to apply heat treatment ata temperature not lower than 300° C., preferably 400° C. or higher butbelow the decomposition temperature of the nitride semiconductor (1200°C.) after forming the second protective film, which makes the secondprotective film less soluble to the material (hydrofluoric acid) thatdissolves the first protective film, thus it is desirable to add thisprocess.

Then the wafer is dipped in hydrofluoric acid to remove the firstprotective film 161 by the lift-off process. Thus the first protectivefilm 161 provided on the p-type contact layer 111 is removed thereby toexpose the p-type contact layer. The second protective film 162 isformed on the side faces of the ridge stripe and the plane whichcontinues therefrom (exposed surface of the p-type optical guide layer109) as shown in FIG. 1.

After the first protective film 161 provided on the p-type contact layer112 is removed as described above, a p-type electrode 120 made of Ni/Auis formed on the surface of the exposed p-type contact layer 111 asshown in FIG. 1. The p-type electrode 120 is formed with stripe width of100 μm over the second protective film 162 as shown in FIG. 1. Afterforming the second protective film 162, an n-type electrode 121 made ofTi/Al in stripe configuration is formed in a direction parallel to thestripe on the n-type contact layer 103 which has been already exposed.

Then the surface of a desired region which has been exposed by etchingwhere lead-out electrodes for the p-type and n-type electrodes are to beformed is masked, and a multi-layered dielectric film 164 made of SiO₂and TiO₂ are formed. Lead-out electrodes 122, 123 made of Ni—Ti—Au (1000Å-1000 Å-8000 Å) are formed on the p-type and n-type electrodes. At thistime, the active layer 107 is formed with a width of 200 μm (width inthe direction perpendicular to the resonator direction). Themulti-layered dielectric film made of SiO₂ and TiO₂ are formed also onthe resonator surface (reflector side).

After forming the n-type and p-type electrodes as described above, thewafer is divided into bar shape along M plane (M plane of GaN, (11-00)or the like) of the nitride semiconductor in the direction perpendicularto the striped electrode. The wafer of bar shape is further divided toobtain laser devices with the resonator length being 650 μm.

The laser device made as described above has the staking structure 20shown in FIG. 7 and shows the band gap energy diagrams and correspondsto the first, second, fourth and fifth embodiments. When dividing thewafer into bars, the wafer may be cleaved along the waveguide interposedbetween etched end faces with the cleavage surface being used as theresonator surface, or may be cleaved at a position other than thewaveguide with the etched end faces being used as the resonator surface,or one of the etched end faces and a cleavage surface may be used as apair of resonator surfaces. While a reflecting film made of amulti-layered dielectric film is provided on the resonator surface ofthe etched end faces, a reflecting film may also be provided on theresonator surface of the cleavage surface after cleaving. The reflectingfilm may be made of at least one selected from among a group consistingof SiO₂, TiO₂, ZrO₂, ZnO, Al₂O₃, MgO and polyimide. The reflecting filmmay also comprise multiple films each having a thickness of λ/4n (λ isthe wavelength and n is the refractive index) stacked one on another, ormay comprise a single layer, and may also be made function as a surfaceprotective film which prevents the resonator end faces from beingexposed, as well as the reflecting film. To function as a surfaceprotective film, the film may be made in a thickness of λ/2n. Such alaser device may also be made as only the n-electrode forming surface(m-type contact layer) is exposed without forming the etching end facein the device manufacturing process, and a pair of cleavage surfaces areused as the resonator surfaces.

When dividing the bar-shaped wafer, too, cleavage surface of the nitridesemiconductor (single substrate) may be used. Alternatively, the bar maybe cleaved in M plane and A plane ({1010}), of the nitride semiconductor(GaN) perpendicular to the cleavage surface which is made when cleavinginto the bar, approximated by hexagonal system, thereby to obtain chips.Also the A plane of the nitride semiconductor may be used when cleavinginto bars.

A laser device capable of continuous oscillation at 405 nm with outputpower of 5 to 30 mW and threshold current density of 2.8 kA/cm² at theroom temperature can be made. The laser device thus obtained haslifetime of 2000 to 3000 hours which is two to three times that ofComparative Example 1 operating with continuous oscillation with outputof 5 mW at 60° C. Reverse withstanding voltage of this device wascompared with that of Comparative Example 1. Many of the laser deviceswere not destroyed and when the voltage was raised to 100 V, some werenot destroyed showing the reverse withstanding voltage characteristicabout twice higher than that of Comparative Example 1.

EXAMPLE 2

Laser devices are made similarly to Example 1 except for the barrierlayers located in the interface between the active layer and the p-typeelectron confinement layer (last-stacked barrier layer and barrier layernearest to the p side), among the barrier layers provided in the activelayer, are doped Mg in concentration of 1×10¹⁸/cm³. The laser devicethus obtained has the last barrier layer doped with Mg more heavily thanin the case of Example 1, and has lifetime and reverse withstandingvoltage characteristic of similar level.

EXAMPLE 3

Laser devices are made similarly to Example 1 except for the activelayer is formed as described below.

(Active Layer 107)

A barrier layer (B) made of In_(0.05)Ga_(0.95)N doped with Si in aconcentration of 5×10¹⁸/cm³ is formed to a thickness of 140 Å at atemperature of 800° C. by using TMI (trimethyl indium), TMG and ammoniaas the stock material gas and silane gas as impurity gas. Then thesupply of silane gas is stopped and a well layer (W) made of undopedIn_(0.1)Ga_(0.9)N is formed to a thickness of 40 Å, while stacking thebarrier layer (B) and the well layer (W) in the order of(B)/(W)/(B)/(W). Last, the last barrier layer made of undopedIn_(0.05)Ga_(0.95)N is formed by using TMI (trimethyl indium), TMG andammonia as the stock material gas. The active layer 107 becomes multiplequantum well structure (MQW) having total thickness of 500 Å.

A laser device capable of continuous oscillation at a wavelength of 405nm with output power of 5 to 30 mW and threshold current density of 2.8kA/cm² at the room temperature can be made. The laser device thusobtained has lifetime of 5000 to 6000 hours in operation of continuousoscillation with output of 5 mW at 60° C. Reverse withstanding voltageof this device was compared with that of Comparative Example 1. This isequivalent to a lifetime near 100 thousand hours at the roomtemperature. The reverse withstanding voltage is about 45V.

EXAMPLE 4

Laser devices are made similarly to Example 1 except that the barrierlayers in the interface between the active layer and the p-type electronconfinement layer (last barrier layer), among the barrier layersprovided in the active layer, are doped Mg in concentration of1×10¹⁸/cm³. The laser device thus obtained has the last barrier layerdoped with Mg more heavily than in the case of Example 1, and haslifetime and reverse withstanding voltage characteristic of similarlevel.

EXAMPLE 5

In Example 1, thickness of the well layer is set to 55 Å. The laserdevice has lifetime significantly longer than that of Example 1, lastingfor 1000 to 2000 hours in continuous oscillation with output power of 30mW at 50° C.

When thickness of the well layer is increased to 60, 80 and 90 Å inExample 1, lifetime of the device rends to increase roughly inproportion to the thickness. At the same time, increases in the value ofVf and threshold current are observed as a result of the increase in thetotal thickness of the active layer as the well layer becomes thicker.In any of these cases, however, very long lifetime of the device isachieved in comparison to Comparative Example 1. With regards to Vf andthreshold current, while definite conclusion cannot be drawn becausethese characteristics are related to the total thickness of the activelayer and depend on the stacking structure, these characteristics do notdepend much on the change in the thickness of the well layer in case thenumber of well layers is 2, which is the least number of well layers inmultiple quantum well structure, and increases in the value of vf andthreshold current are kept at low levels, namely insignificant amountsof increase over Example 1, never resulting in a serious degradation ofthe device characteristics during continuous oscillation of the LD. Tosum up, device characteristics can be improved when the thickness of thewell layer is 40 Å or larger, and device lifetime can be increasedsignificantly when the thickness is 50 Å or larger. When the thicknessof the well layer is 50 Å or larger, oscillation with output power of 80mW can be achieved with some devices achieving an output of 100 mW.

EXAMPLE 6

When the thickness of the last barrier layer (barrier layer located atthe topmost position) is increased to 150 Å in Example 1, a devicehaving lifetime longer than that of Example 1 was obtained. This issupposedly because, as shown in FIGS. 9A and 9B, the increasingthickness of the topmost barrier layer 2 c, which means increasingdistance d_(B) between the well layer 1 b and the p-type electronconfinement layer 28, keeps the well layer from the first p-type nitridesemiconductor layer (p-type electron confinement layer) that has a highresistance and is expected to be heated to a higher temperature than theother layers during operation of the device, thereby protecting the welllayer from the adverse effect of the high temperature and allows laseroscillation with better oscillation characteristics.

EXAMPLE 7

In Example 1, the active layer is made in such a structure as barrierlayer, well layer, barrier layer and well layer stacked in this orderwith the barrier layer having thickness of 70 Å, with a harrier layer140 Å thick provided at the end. Lifetime of the device when operatedunder conditions of continuous oscillation with output of 30 mW at 50°C. is shown in FIG. 12 with the thickness of the well layer beingchanged as 22.5 Å, 45 Å, 90 Å and 130 Å. As will be clear from thedrawing, the thicker the well layer becomes, the longer the lifetime ofthe device, thus making a laser device of longer lifetime. When thethickness of the well layer is 22.5 Å, 45 Å, 90 Å or 130 Å, oscillationof 30 mW or high output power can be made similarly to the case ofExample 5, and laser device having output power of 80 to 100 mW can beachieved when the thickness is 90 Å or 130 Å.

EXAMPLE 8

In Example 1, the active layer is made in such a structure as barrierlayer, well layer, barrier layer and well layer stacked in this orderwith the well layer having thickness of 45 Å, with a barrier layer 140 Åthick provided at the end. Lifetime of the device when operated underconditions of continuous oscillation with output of 30 mW at 5° C. isshown in FIG. 13 with the thickness of the barrier layer other than lastbarrier being changed as 22.5 Å, 45 Å, 90 Å and 130 Å. As will be clearfrom the drawing, when the thickness of the barrier layer is increased,device lifetime remains substantially constant with the thickness around50 Å and larger. Thus satisfactory device lifetime can be made of thenitride semiconductor device of the present invention when the barrierlayer 40 Å or higher.

EXAMPLE 9

Laser devices are obtained similarly to Example 1 except for setting theproportion of Al in the AlGaN layer of the multi-layered cladding layerto 0.1. The laser devices obtained have mean proportion of Al being 0.05in the cladding layer, with self-excited oscillation being observed insome of them during continuous oscillation in the single mode with 30mW. When the proportion of Al in the AlGaN layer of the multi-layeredcladding layer to 0.15, mean proportion of Al in the cladding layerbecomes 0.78, and the probability of self-excited oscillation to occurbecomes higher than in the case where mean proportion of Al is 0.05.Thus a laser device free of self-excited oscillation can be obtained bysetting the proportion of Al in the cladding layer to 0.05 or lower,preferably 0.025 or lower, or 0.03 or lower.

EXAMPLE 10

Laser devices are obtained similarly to Example 1 except for growing thetopmost barrier layer (barrier layer disposed nearest to the p-typelayer side) to a thickness of 150 Å. The laser device thus obtainedshows a tendency of the device lifetime becoming slightly longer thanExample 1. On the contrary, the laser device having the topmost barrierlayer of 100 Å in thickness has lifetime significantly shorter than thatof Example 1.

EXAMPLE 11

Laser devices are obtained similarly to Example 1 except for providingthe p-type optical guide layer 109 directly on the active layer 107without providing the p-type electron confinement layer 108. The laserdevice thus obtained has the value of Vf about 1V lower, although thethreshold current increases sharply and some of the laser devices aredifficult to oscillate. This is supposedly because the absence of thefirst p-type nitride semiconductor layer (p-type electron confinementlayer 108) of high resistance decreases the value of Vf and makes itdifficult to confine electrons in the active layer, thus leading tosharp increase in the threshold.

EXAMPLE 12

Laser devices are obtained similarly to Example 1 except for making theactive layer in a stacked structure of three well layers and fourbarrier layers. The laser device thus obtained has the value of Vfhigher than that of Example 1 because the active layer becomes thickeras a whole, and the threshold current also becomes slightly higher dueto the larger number of the well layers. When the active layer is madeby stacking five barriers and four well layers alternately with abarrier layer at the end, the threshold current the value of Vf becomehigher than in the case where the number of well layers is 2 or 3.

In FIG. 4, when the first barrier layer (second n side barrier layer) 2a and the last barrier layer (first p side barrier layer) 2 d are grownto thickness of 140 Å and the barrier layers 2 b, 2 c are grown tothickness of 100 Å (structure of the active layer shown in FIG. 5),variations in the device characteristics, particularly variations in thelifetime among the chips become less than in the case shown in FIG. 5,thus providing laser devices of better device characteristics.

COMPARATIVE EXAMPLE 1

Laser devices are obtained similarly to Example 1 except that allbarrier layers in the active layer are doped with Si. The laser devicesthus obtained have lifetime of 1000 hours in continuous oscillation withoutput power of 5 mw at 60° C. In the evaluation of the reversewithstanding voltage, most of the Laser devices thus obtained aredestroyed when subjected to reverse voltage of 50V. Device lifetime andreverse withstanding voltage of the devices having the last barrierlayer in the active layer being doped with Si in concentrations of1×10¹⁷/cm³, 1×10¹⁸/cm³ and 1×10¹⁹/cm3 are shown in FIGS. 14, 15. In thegraph, the plot noted as undoped corresponds to the data of Example 1.As will be clear from the graphs, doping the barrier layer disposednearest to the p-type layer side in the active layer with Si causes thedevice lifetime and reverse withstanding voltage to decrease, resultingin degradation of the device characteristic in proportion to theconcentration of doping.

In an analysis of the laser devices thus obtained with SIMS (secondaryion mass spectroscopy analysis), Si and Mg are detected in the topmostbarrier layer disposed in the interface with the p-type electronconfinement layer among the barrier layers in the active layer (barrierlayer located at a position nearest to the p-type layer). Thus the laserdevices obtained has the topmost barrier layer doped with Si and Mg,which is believed to be the cause of the significantly lowercharacteristics than the laser device obtained in Example 1. However, asshown in FIGS. 14, 15, concentration of Mg doping does not change whenthe concentration of Si doping is changed, the degradation in the devicecharacteristics is supposedly caused mainly by n-type impurity.

EXAMPLE 13

In Example 1, laser devices are obtained by using an active layer 407described below with reference to FIG. 10 instead of the active layer107.

(Active Layer 407)

By setting the temperature to 880° C., a first barrier layer 401 a madeof In_(0.01)Ga_(0.99)N doped with Si in a concentration of 5×10¹⁸/cm³ isformed to a thickness of 100 Å by using TMI, TMG and ammonia as thestock material gas and silane gas as the impurity gas. Then with thetemperature lowered to 820° C., the supply of silane gas is stopped anda well layer 402 a made of undoped Ino_(0.3)Ga_(0.7)N is formed to athickness of 50 Å. At the same temperature, a second barrier layer 403 amade of undoped Al_(0.3)Ga_(0.7)N is formed to a thickness of 10 Å usingTMA. The 3-layer structure of the first barrier layer 401 a, the welllayer 402 a and the second barrier layer 403 a is repeated so as tostack layers 401 b, 402 b and 403 b, with a topmost barrier layer 404made of undoped In_(0.01)Ga_(0.99)N being formed to a thickness of 140 Åat the end, thereby forming an active Layer 407 of multiple quantum wellstructure (MQW) having total thickness of 460 Å. At this time, p-typeimpurity Mg diffuses from the adjacent p-type electron confinement layer108 into the topmost barrier layer 404 located at a position nearest tothe p-type layer, thus making the barrier layer which includes Mg. Thusa laser device having high output power and long lifetime and emittinglight of wavelength 470 nm is obtained. At this time, the second barrierlayer provided on top of the well layer is made of a nitridesemiconductor which includes Al, preferably a nitride semiconductorhaving a composition of Al_(z)Ga_(1-z)N (0<z≦1), which is supposed tohave an effect of forming proper unevenness in the well layer and causesegregation of In or distribution in concentration, thus resulting inquantum dot or quantum wire, thereby providing a nitride semiconductordevice having higher output power than the case without the secondbarrier layer. At this time, proper unevenness tends to be formed in thewell layer when the proportion z of Al is not less than 0.3. Similareffect can be achieved when the second barrier layer is provided not incontact with the well layer. It is also preferable that the barrierlayer provided below the well layer in contact therewith does notinclude Al as in the case of the first barrier layer, because thisenables it to form the well layer with good crystallinity.

EXAMPLE 14

A light emitting device shown in FIGS. 9A and 9B is manufactured asfollows.

A substrate made of sapphire (C plane) is set in a MOVPE reactionvessel, with the substrate temperature raised to 1050° C. while flowinghydrogen, and the substrate is cleaned.

(Buffer Layer 302)

With the temperature lowered to 510° C., a buffer layer 302 made of GaNis formed to a thickness of 150 Å on the substrate 301 by using hydrogenas the carrier gas, and ammonia, TMG (trimethyl gallium), TMA (trimethylaluminum) as the stock material gas.

(Base Layer 303)

After growing the buffer layer 302, supply of only the TMG is stoppedand the temperature is raised to 1050° C. At the temperature of 1050°C., a base layer 303 made of undoped GaN is grown to a thickness of 1.5μm using TMG and ammonia gas as the stock material gas. The base layerserves as the substrate whereon to grow the nitride semiconductor.

(n-type Contact Layer 304)

Then an n-type contact layer 304 made of GaN doped with Si inconcentration of 4.5×10¹⁸/cm³ is formed to a thickness of 2.25 μm at atemperature of 1050° C. by using TMG and ammonia as the stock materialgas, and silane gas as an impurity gas.

(n-type First Multi-Layered Film Layer 305)

Then with supply of only the silane gas stopped, abase layer 305 a madeof undoped GaN is formed to a thickness of 3.000 Å at a temperature of1050° C. using TMG and ammonia gas, followed by the growth of anintermediate layer 305 b made of GaN doped with Si in concentration of4.5×10¹⁸/cm³ to a thickness of 300 Å at the same temperature by addingthe silane gas. Then again with supply of only the silane gas stopped,an upper layer 305 c made of undoped GaN is formed to a thickness of 50Å at the same temperature, thereby forming a first multi-layered filmlayer 305 consisting of three layers 304 a, 305 b and 304 c with totalthickness of 3350 Å.

(n-type Second Multi-Layered Film Layer 306)

Then at roughly the same temperature, the second nitride semiconductorlayer made of undoped GaN is grown to a thickness of 40 Å, followed bythe growth of the first nitride semiconductor layer made of undopedIn_(0.13)Ga_(0.87)N to a thickness of 20 Å at 800° C. using TMG, TMI andammonia. This operation is repeated to stack the second nitridesemiconductor layer and the first nitride semiconductor layeralternately ten times in this order. Last, the second nitridesemiconductor layer made of GaN is grown to a thickness of 40 Å, therebyforming the n-type second multi-layered film layer 306 of super latticestructure having thickness of 640 Å.

(Active Layer 307)

A barrier Layer made of GaN is grown to a thickness of 250 Å, followedby the growth of well layer made of undoped In_(0.3)Ga_(0.7)N to athickness of 30 Å at 800° C. using TMG, TMI and ammonia. By stackingseven barrier layers and six well layers alternately in such aconstitution as barrier layer B₁/well layer/barrier layer B₂welllayer/barrier layer B₃/well layer/barrier layer B/well layer/barrierlayer B₅/well layer/barrier layer B₆/well layer/barrier layer B₇,thereby forming the active layer 307 of multi-quantum well structurehaving total thickness of 1930 Å. At this time, the barrier layers B₁,B₂ are doped with Si in concentration of 1×10¹⁷/cm³ and the otherbarrier layers B. (i=3, 4, . . . ,7) are grown undoped.

(p-type Multi-Layered Cladding Layer 308)

A third nitride semiconductor layer made of Al_(0.2)Ga_(0.8)N doped withMg in a concentration of 1×10²⁰/cm³ is formed to a thickness of 40 Å at1050° C. by using TMG, TMA, ammonia and Cp₂Mg (cyclopentadienylmagnesium) as the impurity gas. Then with the temperature being set to800° C., a fourth nitride semiconductor layer made ofIn_(0.03)Ga_(0.97)N doped with Mg in a concentration of 1×10²⁰/cm³ isgrown to a thickness of 25 Å by using TMG, TMI, ammonia and Cp₂Mg. Theseoperations are repeated to stack the third nitride semiconductor layerand the fourth nitride semiconductor layer alternately five times inthis order, with one layer of the third nitride semiconductor layerhaving thickness of 40 Å being grown at the end. Thus the p-typemulti-layered cladding layer 308 of super lattice structure havingthickness of 365 Å is formed.

(p-type GaN Contact Layer 310)

Then with the temperature being set to 1050° C., a p-type contact layer310 made of p-type GaN doped with Mg in a concentration of 1×10²⁰/cm³ isgrown to a thickness of 700 Å by using TMG, ammonia and Cp₂Mg.

Upon completion of the reaction, the temperature is lowered to the roomtemperature, and the wafer is annealed at 700° C. in nitrogen atmospherewithin the reaction vessel, thereby to decrease the resistance of thep-type layer.

After annealing, the wafer is taken out of the reaction vessel. A maskof a predetermined shape is formed on the surface of the p-type contactlayer 310 provided on the top, and the p-type contact layer is etched inan RIE (reactive ion etching) apparatus, thereby exposing the n-typecontact layer 4 as shown in FIGS. 9A and 9B.

After etching, a p-type electrode 311 which transmits light made of amaterial including Ni and Au is formed to a thickness of 200 Å oversubstantially the entire surface of the p-type contact layer 310provided on the top, and a p-type pad electrode made of Au for bondingis formed to a thickness of 0.5 μm on the p-type electrode 11. On theother hand, an n-type electrode 312 which includes W and Al is formed onthe surface of the n-type contact layer 304 which has been exposed byetching, thereby to obtain a light emitting device. In the lightemitting device-thus obtained, since the barrier layer B₁ which islocated nearest to the n-type layer and the next barrier layer B₂ aredoped with an n-type impurity, the carrier from the n-type layer isefficiently injected deep into the active layer (toward the p-typelayer), thereby improving the photo-electric conversion efficiency,decreasing the value, of Vf and the leak current, and increasing thelight emission output power compared to Comparative Example 2 whereinall the barrier layers are grown undoped.

EXAMPLE 15

Light emitting devices are obtained similarly to the Example 14 exceptthat the barrier layer B₇ located nearest to the p-type layer is dopedwith Mg as a p-type impurity in concentration of 1×10¹⁸/cm³ in theExample 14. In the light emitting device thus obtained, since thetopmost barrier layer B₇ has a p-type impurity, the carrier from thep-type layer is efficiently injected, thereby improving thephoto-electric conversion efficiency, and increasing the light emissionoutput power compared to Example 14.

COMPARATIVE EXAMPLE 2

Light emitting devices are obtained similarly to Example 14 except thatall the barrier layers and the well layers in the active layer are grownundoped in Example 14. The light emitting devices thus obtained havelower light emission output power and shorter lifetime than those ofExample 14.

EXAMPLE 16

A plane emission type laser device shown in FIG. 11 will be describedbelow.

(Substrate 501)

A substrate 501 similar to the nitride semiconductor substrate 101 usedin Example 1 is used.

A reflecting film 530 is formed by stacking a first layer 531 made ofAlN and a second layer 532 made of GaN alternately three times on thenitride semiconductor substrate 501. Each of the stacked layers isformed to a thickness of λ/4n (λ is the wavelength and n is therefractive index) wherein n is set to 2 (AlN) or 2.5 (GaN) with thethickness being about 500 Å for the first layer and 400 Å for the secondlayer. At this time, the reflecting film may comprise the first and thesecond layers made of nitride semiconductor represented byIn_(x)Al_(y)Ga_(1-x-y)N (0≦x≦1, 0≦y≦1, x+y=1), while the first and thesecond layers of the reflecting film made of nitride semiconductor arepreferably multi-layered film formed by stacking nitride semiconductorsof different compositions represented by Al_(x)Ga_(1-x)N (0≦x≦1). Atthis time, each layer is formed once or more, and one or more pairs ofthe first layer and the second layer are formed. Specifically, the pairof the first layer and the second layer may be formed in such aconstitution as AlGaN/AlGaN, GaN/AlGaN, AlGaN/AlN or GaN/AlN.Composition of Al_(x)Ga_(1-y)N/Al_(y)Ga_(1-y)N (0<x, x<y<1) is amulti-layered film of AlGaN and is therefore capable of decreasing thedifference in the thermal expansion coefficient and achieve goodcrystallinity. Composition of GaN/Al_(y)Ga_(1-y)N (0<y<1) makes itpossible to form a multi-layered film of GaN layer having improvedcrystallinity. When the difference in the proportion of Al (y-x) isincreased, difference in the refractive index between the first layerand the second layer increases resulting in a higher reflectivity.Specifically, a multi-layered reflecting film of high reflectivity canbe made by setting y-x≧0.3, preferably y-x≧0.5. Also similarly toExample 1, the multi-layered film made of Al_(y)Ga_(1-y)N (0<y≦1)functions as the buffer layer 102 and provides an effect of decreasingpits.

Then an n-type contact layer 533, an active layer 534, a p-type electronconfinement layer (not shown) and a p-type contact layer 535 are stackedunder conditions similar to those of Example 2 (well layer 55 Å), and ablocking layer 536 made of SiO₂ having a circular aperture is provided.A second p-type contact layer 537 is formed by growing Mg-doped GaNthrough the circular aperture. At this time, either one of the p-typecontact layer 535 and the second p-type contact layer 537 may be formed.A multi-layered dielectric film made of SiO₂/TiO₂ is formed on thesecond p-type contact layer 537 to make a reflecting film 538, which isprovided on the aperture of the blocking layer 536 in a circularconfiguration. Then the p-type contact layer 535 is exposed by etching,and a ring-shaped r-type electrode 421 is formed on the p-type contactlayer 535 which has been exposed and a p-type, electrode 520 whichsurrounds the reflecting film 538 is formed on the second p-type contactlayer 537. The planar emission type laser device thus obtained hasexcellent device lifetime and high output power similar to that ofExample 2.

EXAMPLE 17

Laser devices are obtained similarly to Example 1 except that the activelayer and the p-type cladding layer are formed as described below.

(Active Layer 107)

A barrier layer (B) made of In_(0.05)Ga_(0.95)N doped with Si in aconcentration of 5×10¹⁸/cm³ is formed to a thickness of 70 Å. Then thesupply of silane gas is stopped and a well layer (W) made of undopedIn_(0.15)Ga_(0.9)N is formed to a Thickness of 70 Å, while stacking thebarrier layer (B) and the well layer (W) in the order of (B)/(W)(B)/(W). Last, topmost barrier layer made of undoped In_(0.05)Ga_(0.95)Nis formed to a thickness of 150 Å by using TMI (trimethyl indium) as thestock material gas. The active layer 107 becomes multiple quantum wellstructure (MQW) having total thickness of 430 Å.

(p-type Cladding Layer 110)

Then layer of undoped Al_(0.10)Ga_(0.95)N is formed to a thickness of 25Å and a layer of Mg-doped GaN is formed to a thickness of 25 Å. Thisoperation is repeated 90 times to form the p-type cladding layer 110constituted from super lattice structure of total thickness of 0.45 μm.

In the laser device obtained as described above, ratio Rt of the barrierexcept for the barrier layer located nearest to the p-type layer in theactive layer and the well layer is 1, satisfying the relationshipbetween the well layer thickness and the device lifetime shown in FIG.12. Thus a laser device having high output power and long lifetime canbe provided. Also as the thickness of the barrier layer (the n sidebarrier layer or the barrier layer sandwiched by the well layers) isdecreased, a laser device having excellent response characteristic andRIN is provided for an optical disk system. Also as the proportion of Alin the p-type cladding layer is increased, difference in the refractiveindex thereof from that of the buried layer 162 becomes smaller,resulting in a laser device of effective refractive index type havingless confinement in the lateral direction which is free from kink evenin a high output region. It is preferable to provide the p-type claddinglayer which has the mean composition of Al_(x)Ga_(1-x)N in meanproportion x being in a range of 0<x≦0.1. This suppresses the kinkfromation in the laser device.

According to the present invention, a nitride semiconductor devicehaving excellent device lifetime and high output power can be obtainedwhere the reverse withstanding voltage, of which weak characteristic ofa device made of nitride semiconductor has been a problem in the priorart, is improved. When the nitride semiconductor device of the presentinvention is applied to laser device, a laser device having the improvedcharacteristics similarly to those described above and is free fromself-excited oscillation can be obtained.

What is claimed is:
 1. A nitride semiconductor device wherein an activelayer is sandwiched between p-type nitride semiconductor layers andn-type nitride semiconductor layers, wherein said p-type nitridesemiconductor layers has an electrons confining layer adjoining saidactive layer and made of nitride semiconductor that includes Al; andsaid active layer has a quantum well structure including at least onewell layer made of nitride semiconductor that includes In and barrierlayers made of nitride semiconductor, wherein a first barrier layerarranged in the nearest position to said p-type nitride semiconductorlayer among said barrier layers substantially does not have an n-typeimpurity, while a second barrier layer that is different from said firstbarrier layer has an n-type impurity.
 2. The nitride semiconductordevice according to claim 1, wherein the film thickness of said firstbarrier layer is greater than the film thickness of said second barrierlayer.
 3. The nitride semiconductor device according to claim 1, whereinsaid active layer has L (L≧2) barrier layers so that the barrier layerarranged in a position nearest to said n-type nitride semiconductorlayer is denoted as barrier layer B₁ and the i-th barrier layer (i=1, 2,3, . . . L) counted from the barrier layer B₁ toward said p-type nitridesemiconductor layer is denoted as barrier layer B_(i); and barrierlayers B_(i) from i=1 to i=n (1<n<L) include an n-type impurity.
 4. Thenitride semiconductor device according to claim 1, wherein the entirebarrier layers other than said first barrier layer include an n-typeimpurity.
 5. The nitride semiconductor device according to claim 1,wherein said first barrier layer is arranged in the outermost positionin said active layer.
 6. The nitride semiconductor device according toclaim 1, wherein said second barrier layer is arranged in the outermostposition close to said n-type nitride semiconductor layer within saidactive layer.
 7. The nitride semiconductor device according to claim 6,wherein the film thickness of said first barrier layer is approximatelythe same as the film thickness of said second barrier layer.
 8. Thenitride semiconductor device according to claim 7, wherein said activelayer has 2 or more well layers and has a third barrier layer betweenthe well layers; and the film thickness of said third barrier layer issmaller than the film thickness of said first p side barrier layer andsaid second n side barrier layer.
 9. The nitride semiconductor deviceaccording to claim 1, wherein at least one well layer within said activelayer has a film thickness of not less than 40 Å.
 10. The nitridesemiconductor device according to claim 1, wherein said first barrierlayer has a p-type impurity.
 11. The nitride semiconductor deviceaccording to claim 1, wherein said first barrier layer includes a p-typeimpurity in the range of no less than 5×10¹⁶ cm⁻³ and no more than1×10¹⁹ cm⁻³.
 12. The nitride semiconductor device according to claim 1,wherein said first barrier layer is p-type or i-type.
 13. The nitridesemiconductor device according to claim 12, wherein said first barrierlayer has been grown without being doped with an impurity and includes ap-type impurity through diffusion from said p-type nitride semiconductorlayer.
 14. The nitride semiconductor device according to claim 1,wherein said n-type nitride semiconductor layer, said active layer andsaid p-type nitride semiconductor layer are layered in sequence.
 15. Thenitride semiconductor device according to claim 1, wherein said p-typenitride semiconductor layer has an upper clad layer made of a nitridesemiconductor that includes Al of which the average mixed crystal ratiox is in the range of 0<x≦0.05; said n-type nitride semiconductor layerhas a lower clad layer made of a nitride semiconductor that includes Alof which the average mixed crystal ratio x is in the range of 0<x≦0.05;and the nitride semiconductor device has a laser device structure. 16.The nitride semiconductor device according to claim 1, wherein saidfirst p-type nitride semiconductor layer is provided so as to contact abarrier layer nearest to said p-type nitride semiconductor layer and hasbeen grown being doped with a p-type impurity of which concentration ishigher than that of said barrier layer in said active layer.
 17. Thenitride semiconductor device according to claim 1, wherein the number ofwell layers in said active layer is from 1 to
 3. 18. The nitridesemiconductor device according to claim 1, in said active layer saidsecond barrier layer is arranged between well layers and the filmthickness ratio Rt (=[film thickness of a well layer]/[film thickness ofa barrier layer]) of said well layer to the second barrier layer is inthe range of 0.5≦RT≦3.
 19. The nitride semiconductor device according toclaim 1, wherein the film thickness dw of said well layer is in therange of 40 Å≦dw≦100 Å while the film thickness of db of said secondbarrier layer is in the range of db≧40 Å.
 20. The nitride semiconductordevice according to claim 1, wherein said p-type nitride semiconductorlayer has an upper clad made of a nitride semiconductor that includes Aland said n-type nitride semiconductor has a lower clad layer made of anitride semiconductor, wherein the average mixed crystal ratio of Al inthe upper clad layer is greater than that of the lower clad layer. 21.The nitride semiconductor device according to claim 20, wherein theaverage mixed crystal ratio x of Al in said upper clad layer is in therange of 0<x≦0.1.
 22. The nitride semiconductor device according toclaim 1, wherein the active layer has a well layer of which distance dBfrom the first p-type nitride semiconductor layer is in the range of noless than 100 Å and no more than 400 Å and has a first barrier layerwithin the distance dB.